Looking briefly at routes from Burton-on-Trent to London, it seems the routeing points are now Derby, Nottingham and Tamworth ... but not Birmingham Group. It seems to me that in the past an "any permitted route" ticket allowed me to travel via New Street into Euston or (useful before of fewer restrictions on the return journey) to go into New Street, walk to Moor Street, and take a Chiltern line service into Marylebone.
Tamworth has never been a terribly useful option because it has no regular fast service into London, and even the new London Midland has a three-hour gap in the afternoon and no trains back after 1900h. On the other hand I suppose going via Nottingham gives one more choice from St Pancras.
But can someone confirm that my interpretation is correct: i.e., no more travel via B'ham?
Routeing permission WM+TV would give Tamworth-Birmingham on map WM, the down the west coast into Euston or Kensington Olympia. It would not be valid via the Chilterns.
I am surprised that Nottingham is a routeing point for Burton, as you would virtually always have to go via Derby to get there.