Then she can get a Non O visa ‘for Retirement’. Pretty easy from London. It would give her 90 days that she extend for another 30 days at Thai Immigration.
She could even stay 15 months, though that needs money in thai bank (unless you pay an agent) and use Extensions to visit each year without getting another visa. (It would mean paying an agent 12-15k or so each year).
I’m not sure if they’ll give the visa if your flight to Thailand is not out of the U.K. You can ask the Embassy and see, but I have my doubts if you are currently resident in another country.
The Visa Exempt entry has nothing specifically to do with the Sandbox, it’s a general thing. You can extend your stay by a further 30 days at immigration in Thailand.
Currently, the 45 days on entry instead of the usual 30 is meant to last until the end of September. However, given there’s no chance of quarantine ending, it’s very likely to continue to be 45 days for quite some time.