I think the answer to the question "what would you run" is actually pretty simple. Buses.
You can speculate as much as you want about reallocating withdrawn stock and clever timetable adjustments, but ultimately that's a lot of hassle and cost for the TOC - almost certainly more than just paying for road transport.
Though they're not even doing that.
Capacity could be an issue (particularly with COVID), so I think a sensible plan would be to get buses in and run them on a very unattractive timetable, i.e. all mainline stations (but frequent), and only as far as Reading/Didcot for connections on TfL Rail etc rather than all the way. And still offer (and encourage) tickets to be reused on other days, full refunds etc. Perhaps they could even consider offering
more than a refund, e.g. a refund plus a free journey voucher as a real enticement.
For LNER, the same, but run the buses to Peterborough for GA, for example, or even spread it out by running to e.g. Northampton and paying LNR to rustle up some 12 car sets.
So you're offering something so people aren't fully stranded, but it's unattractive so people are more likely to refund or reschedule.