It's a valid route yes but don't the London Underground need to see the Maltese Cross to accept validity? As I said, in my example they were quite insistent that it was not valid even though the route clearly involved using the London Underground.
Routeing Guide said:
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.
The via London, Maltese Cross symbol on a ticket signifies that the ticket
may be used via London Underground or Thameslink services. Unless a ticket
specifies that the journey must be made via London, passengers are free to
use an alternative permitted route for their journey as provided by the
Routeing Guide.
In some instances (particularly long distance cross country journeys) the
Manual will show an "any permitted" fare but without the via London, Maltese
cross symbol. Reference to Section C (the yellow pages) may show via
London to be a permitted route for this journey and in such instances travel
via London to include cross-London transfer would be permitted.
Okay that refers to Any Permitted tickets, however the principle is that tickets do not have to have the maltese cross symbol to be valid via London. They
should have it, but mistakes do occur.
My understanding is that LU get a lump sum for all cross-London journeys rather than be allocated money from any individual sale anyway, but I am not entirely certain about that (not that revenue allocation ever determines validity!)
Paddington is unlikely to be an issue, as the the H&C line platforms are beyond the same gatelines used by FGW services (and if interchanging from high numbered platforms to LU trains on the H&C platforms you do not pass through a barrier).
Euston Square may be an issue though, but that's only one place to argue your case rather than two.
I've done battle at King's Cross before, to get to Paddington (obviously no issue at Paddington), I won by bringing a copy of the relevant documentation such as the page of the Routeing Guide, itinerary & reservations showing travel via LU. One person wasn't co-operative but his colleague told him "he's right, let them through" after a while