Foreign nationals are subject to the ‘Stay at Home’ regulations. You should not travel abroad unless it is permitted. This means you must not go on holiday.
You'll find very little experience of anyone 'currently' flying to Thailand from the UK.
As i understand it, the current UK rules state.
Under current UK COVID-19 restrictions, you must stay at home. You must not travel, including abroad, unless you have a legally permitted reason to do so. It is illegal to travel abroad for holidays and other leisure purposes.