When using a national rail ticket that involves an underground journey across London from one terminus to another, a paper ticket is obviously valid to use on the tube ftom terminus to terminus, but is it valid for any zone 1 journey or just between the specific termini?
So for example could it be used for a Paddington to Victoria journey to go from Lancaster Gate to Green Park instead?
It isn't as simple as being Zone 1 ticket; on the one hand it's valid at a number of stations well outside zone 1 (including Amersham in Zone 9!), but there are also many Zone 1 stations where it isn't valid.
The transfer has to be started at a recognised interchange station -
@jfollows has helpfully provided the full list of these stations above.
Most of these are obvious stations such as London Termini served by both National Rail and Underground/DLR services, but a few are for 'outer London' interchanges (e.g. Wimbledon or Upminster), or are intended to serve nearby National Rail stations (e.g. Tower Hill for Fenchurch Street, or Lancaster Gate for Paddington).
You can then use any reasonable route to transfer between 'appropriate' interchange stations. Obviously that leaves some room for interpretation but I would suggest anything that a popular journey planner comes up (such as TfL Go or their website, National Rail Enquiries or a ticket retailer, CityMapper or Google Maps) would likely be considered reasonable.
You can exit at any intermediate station along your reasonable route, but you cannot continue the transfer after having exited. This point is sometimes contentious, as a notable minority of TfL barrier staff don't seem to be aware of the part of their training manual that states this. However I have rarely been completely refused exit, it usually involves a discussion and being allowed "just this once".
