The most sensible/feasible suggestion is the use of XC HSTs, these are based at Laira so could be used to operate between Reading and Exeter.
LNER are operating a near normal service, with most of their 801s unaffected, so it would make sense to cut LNER services so a few 801s could be borrowed for London to Cardiff services.
Then some northern 150s could be sent down to Bristol to release 166s to be used on Cotswold services and Swindon to Bristol shuttles, maybe cut the Gatwick to Reading services to free up turbos as well.