These are the locations would make sense to keep largely traditional due to the number of foreign tourists etc. in London making complex queries. St Pancras and Kings + are well used throughout the day.
There's a strong case, if designing this properly, for the 20-30 or so key main stations to have significantly sized travel retail facilities - call them "travel centres" or whatever - which would have TVMs, people helping, plus a few desks where advice can be obtained, including on TfL/TfGM/Merseytravel/Nexus etc products. I'm thinking Euston, Kings X/St Pancras (no need for both, they're next to each other, you could even have it in one of those buildings in between the two or one of the street facing retail units on the front of St P which is convenient for both), Paddington, Victoria, Manc Picc, Edinburgh Waverley, Glasgow Central, Liverpool Lime St, Newcastle, Bristol TM and such.
There'd even be an argument for Network Rail providing these (or another contractor) as a wholly impartial GBR facility, not a TOC one, so the experience is consistent.
But because this is being done on a per-TOC basis, we're left with nonsense like Barrow in Furness staying open and Euston closing. Sure, Barrow is probably one of the places where cash is more commonly used, but cash TVMs are possible.