The UK has quite a high number of cases. However the risk of that person bringing covid to Thailand could be reduced by requiring them to do a PCR test 72 hours before departure and again shortly after arrival in Thailand. My guess is those requirements would remain in place in a sandbox programme.
With the testing, I suspect any arrival from the UK is less likely to have covid than the locals. Particularly when the locals will be avoiding testing like the plague, just in case failing gets them sent to a field hospital.
The other point is variants. Currently the UK has delta, just the same as Thailand. However the UK has a very active programme screening for virus mutations, so if something new appears, it is likely to be detected quickly.
In that respect, the UK is low risk.
1 I guess it's a sandbox, which would put a lid on the numbers, as that is not full normality.
2 The choice and timing of flights is currently inferior to pre-covid. So if they do announce some reasonable rules, might we see the likes of EVA dust off their planes & start providing some more choice for the consumer ?