Yes, no problem.
Easier than the NRG and generally correct is to check on National Rail Enquiries, inputting a via point. If shown as valid on one ticket, it's a permitted route (subject to route restrictions on your season).
More recently, if you click "other tickets" on an itinerary, you can view "other valid routes" (doesn't work if you've specified a via point). It's a bit shaky at the moment but a step in the right direction. This shows you're valid via Bristol Parkway or Temple Meads, or via Hereford and Birmingham down into Euston, or via Hereford and Oxford. It misses out the valid route via Bristol and Salisbury to Waterloo, and I think there are others that should be allowed but haven't time to check the NRG now.
