Looking at Route GC. Does that mean I could buy a London to Leamington Spa open rtn via High Wycombe and come back with a stop over in Oxford?
Yes you can, but not based on map GC alone.
If you want to look up permitted routes, don't start with a map, but instead use the routeing guide to define
which maps are applicable.
In the case of London to Leamington, these are
and these define routes from - essentially - Euston, Marylebone and Paddington respectively.
So you can travel out via High Wycombe (map GC) and return via Oxford (map combination RB + RG in the return direction).
The maps are at the end point of the process, effectively, if you need to look up routes which aren't automatically valid such as the shortest route and any route taken by direct trains.
A stop over in Oxford will also be conditioned by the ticket that you hold; it's the exception that break of journey isn't allowed, but there may be route or operator restrictions. In the case of London to Leamington there's a "VIA BANBURY" ticket which would exclude Euston but not Oxford (and also, technically, adds map combination GW+BB to the mix, although this doesn't actually add any additional routes as it happens), and there's an "ANY PERMITTED" ticket which would allow all three routes.
EDIT For the original poster, if "Birmingham Moor Street" was really what was intended - and that's not totally clear - the point was that the valid route "any permitted" from Welwyn to High Wycombe is via London, and then the London to High Wycombe leg is only valid on routes shown on map GC. Which is essentially the obvious one from Marylebone but Paddington-Ealing-High Wycombe is also valid.
