But if it's valid between SRT and Petts Wood, and between SRT and VIC, surely it's valid on fasts from Bromley S to VIC (and BFR)? Or have I missed something?
Shortlands is part of Bromley South Routeing Point group. Mapped routes from Bromley South routeing point group to London include the line via Grove Park as well as the line via Beckenham Junction.
To go via Grove Park, you would have to travel from Shortlands to Petts Wood, and then from Petts Wood to London.
This would not be doubling back or going beyond the origin of the ticket, since any given journey from anywhere on the Beckenham Junction line to Petts Wood can be considered, between Victoria and Shortlands, as the
return leg of a Shortlands - London journey, and then from Shortlands to Petts Wood (or stations in between) part of an outbound Shortlands - London ticket via Grove Park.
Just to illustrate, a train that calls:
Bromley South
Shortlands
Victoria
(only)
between Bromley South and Shortlands you would be travelling on the
return leg of a Shortlands - London season (using the via Grove Park routeing)
and then between Shortlands and Victoria you'd be on the
outbound of the Shortlands - London season (using the via Beckenham Junction routeing).
If on the other hand the train called at
Bromley South
Victoria
only
then you would have started beyond the origin of your ticket, which is Shortlands, and would have no valid ticket for the journey.
The routeing guide permits you to double back within a routeing point group under certain circumstances, and Bromley South and Shortlands are both part of the same routeing point group.
"Some stations are grouped together to improve interchange between trains
by offering customers access to a wider choice of train services and station
facilities. A customer may travel via any station in such a group, including
doubling back, provided that the group is on one of the permitted routes
between their origin and destination stations. This extended availability is for
interchange purposes only and does not apply where the origin or destination
stations are part of a group"
Since Shortlands is part of a group, there is no question of doubling back being permitted here.
Another ticket origin at the same price would be Ravensbourne. Because London single fares are heavily zoned it passes the fares check for Bromley South, andagain travel is permitted to Petts Wood, via Beckenham Junction, and via Catford.
In additiion, since Ravensbourne is not part of Bromley South group, you could travel
Ravensbourne - Bromley South
Bromley South - London Victoria
The routeing guide indicates that doubling back is for interchange purposes only. There is a sort of philosophical issue here:
Ravensbourne - Bromley South
Bromley South - Victoria (non-stop)
doubles back between Bromley South and Shortlands, but is permitted because you are doing it for 'interchange purposes'.
Starting short from Bromley South either is or is not permitted, because on the one hand it's permitted if you do it from Ravensbourne, and in general you can start and stop anywhere on a permitted route, but on the other hand it's not permitted because you are not doubling back for interchange purposes, because you aren't doing an interchange, you are just starting short.
As a side issue, Ravensbourne is linked to Lewisham, and one of the maps from Lewisham to London permits several extra routes
Map CT is deceptively simple
http://www.atoc.org/clientfiles/File/Maps.pdf#page=22
but unlike the other maps, shows not New Cross - London Bridge, but
New Cross - London Routeing Point Group
which is therefore
New Cross - London Bridge
PLUS
London Bridge - Cannon Street
London Bridge - City Thameslink
London Bridge - Charing Cross
since all of of the stations between London Bridge and these respective stations are members of the London Routeing Point group, and therefore are represented on the map as a single square.
Therefore, a Ravensbourne - West Hampstead ticket certainly permits travel to London Bridge and from London Bridge to Blackfriars.
In addition, map NK shows some extra validity on what could be considered 'loophole' routeings
http://www.atoc.org/clientfiles/File/Maps.pdf#page=81
Lewisham - Charlton
Charlton - London Bridge
Lewisham - Dartford (via Eltham)
Dartford - Charlton
There is a slight complication here.
According to the routeing guide. Ravensbourne is not Lewisham, and according to the Routeing Guide, the permitted route
"is the shortest route to the routeing point"
so the shortest route from Ravensbourne to Lewisham
which is clearly either
Ravensbourne - Shortlands
Shortlands - Lewisham via New Beckenham
or
Ravensbourne - Nunhead
Nunhead - Lewisham
I suspect they may both be the same, but it's not important, as neither possible shortest route to Lewisham doubles back with any of paths required to trace these extra routes from Lewisham to London that don't double back.
All potentially quite academic as guards are unlikely to be capable of validating this.....
----
Now.....
The separate question here of whether 'London Not Underground' restricts your chosen London terminal is not clear. If we go back to Shortlands - West Hampsted Stations
Shortlands is part of the Bromley South routeing point group, and West Hampstead Stations is also a routeing point group.
The pemitted routes between Bromley South and West Hampstead are
LONDON.
According to the National Routeing Guide
""If there is a route “LONDON” as one of the permitted routes in the Routeing Guide, the range of permitted routes via London is discovered by the following method:
1. Look up the permitted routes from the origin routeing point to London.
2. Look up the permitted routes from London to the destination routeing point.
3. Work out the range of permitted routes for the whole journey by combining any route found in (1) with any route found in (2). All possible combinations are permitted routes for the journey except those with a repeated map."
"
Mapped routes from Shortlands to West Hampstead Stations are therefore
LK+LB
LK+LD+LB
LK+SL+LB
LR+SL+LB
VC+LB
LK+OV
LK+LD+OV
LK+SL+OV
LR+SL+OV
VC+OV
"If the routeing code is "LONDON", journeys include the cost of cross-London
transfer either by London Underground or Thameslink services. In all cases
the transfer points should be along the correct line of route given by the
‘permitted route’ map combinations. "
With the following example:
"Crossing London from map WX allows the following routes on London Underground:
Waterloo or Victoria or Vauxhall or Richmond to Kings Cross or St Pancras
Waterloo or Victoria or Vauxhall or Richmond to London Liverpool Street
Waterloo or Victoria or Vauxhall or Richmond to Finsbury Park
Once the Underground journey is broken it may not be resumed on the same ticket."
It does say you can take any route on say map LK, plus say any route on map OV.
So for example a routeing is:
Bromley South - Charing Cross
plus
Euston - West Hampstead via Willesden Junction
It's also worth noting this:
"Routes created from the route LONDON in the yellow pages do not oblige you to change in London. For example Oakham to Brighton has the route LONDON which yields LB+MM. This allows the journey Oakham – Leicester – Bedford – Brighton, changing at Bedford and continuing via Thameslink to Brighton."
In addition the National Rail Season Ticket calculator says
"London Underground travel is not included"
so it does in fact seem reasonably clear that a single ticket with route LONDON is therefore valid for two separate journeys, one to, and one from London, and in addition, you may always use Thameslink if it would connect the London terminal that was the mapped destination of the first journey (TO London) and the London Terminal that is the mapped origin of the second journey (FROM London); and, generally,
but not in this case (because of the "London Not Underground" restriction), underground travel between any two stations on the TO London and FROM London journeys.
This seems consistent with the fact of London Underground travel being thrown in as a 'bonus', on tickets without mapped routes, such as Battersea Park - South Hampstead, as I suggested recently here:
http://www.railforums.co.uk/showpost.php?p=1364750&postcount=72
However, cf. this thread
http://www.railforums.co.uk/showthread.php?t=78526
where Brighton - St Pancras is NOT permitted via Victoria, the issue here being that St Pancras is part of the London Routeing Point group, and therefore you do not have a LONDON permitted route, and therefore no cross-London transfer is included. The OP there could have purchased a West Hampstead season to travel to Victoria, albeit it is £2.70/wk more expensive than the St Pancras ticket.
As a final note, validity between Willesden Junction and Clapham Junction on the Overground would appear not to be permitted, since this would require the TO and FROM legs of the ticket to stop at Clapham Junction rather than London.