The U.K Foreign and Commonwealth Office has 2 lists on its website. The first is a change to the travel advisory which had advised against all non essential travel overseas. Thailand is now a Country U.K citizens (in theory) can travel to
‘COVID-19 travel guidance
The Foreign & Commonwealth Office currently advises British nationals against all but essential international travel. Travel to some countries and territories is currently exempted’.
List here;
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The 2nd list is the Countries where you do not have to self isolate for 14 days, if travelling into the U.K from them. Unbelievably Thailand is NOT on this list. So travelling to the U.K from Thailand you must self isolate for 14 days.
They do say the lists will be reviewed;
‘Coronavirus (COVID-19): travel corridors
You do not have to self-isolate on arrival in England if these are the only places you have been to or stopped in during the previous 14 days’.