Just wondering if its possible to break the journey in York outward?
Apart from ECML and WCML , what other routes is it valid?
No, because the restriction code,
5F, bars break of journey on the outward portion.
However, you could get a Super Off-Peak Return from somewhere like Larbert, which is the same price (well, 40p more), and it would then be permitted to go to London via Glasgow and York, and to break your journey en-route. This is because the restriction code,
1K, doesn't bar break of journey.
The hypothetical route from Larbert to London as described above would involve doubling back between Croy and Glasgow (assuming you took a fast E-G via Falkirk High service), but that is permitted in this case.
Most Scotland-London tickets are extremely flexible. You can go on the WCML and ECML on all relevant "side" branches (e.g. via Barrow-in-Furness, Manchester, Birmingham and Northampton if WCML or via Hartlepool, Leeds and Lincoln if ECML). From Peterborough onwards you can go via Cambridge and then into either Liverpool Street, King's Cross, St Pancras or Moorgate.
Further west you can also go via Banbury into Marylebone or Paddington. The former "Thames Clyde" express route, via Dumfries and Settle is permitted. The MML all the way down from Leeds is permitted. It's pretty flexible.
The relevant Routeing Guide maps and map combinations to look up (
here on the map definition page) are shown on page 691 of the
Yellow Pages.
Unfortunately the RDG and the train companies (and sadly the organisations supposed to protect passengers' interests, such as Transport Focus) think that it is complicated for passengers to be given a choice of different routes. They are all working, with the DfT's blessing, to try and reduce the number of permitted routes. In the case of Edinburgh-London you can now no longer buy a fully flexible standard class ticket, being tied to either one routed "via York" or one routed "Avanti".