Do I need a visa for my stay in Thailand if I have a Thailand Pass?

From the Uk you can enter Thailand with no visa and you’d be given 30 days on entry. You can extend that once for a further 30 days. You could buy a tourist visa from the London embassy prior to arrival and that would give you 60 days on arrival, which is also extendable once for 30 days.

Be aware that you may well need a flight out of Thailand within the days allowed of your visa (or exempt). Generally it’s up to the airline whether they’ll allow you to board if you don’t. Some get away with it. Many don’t.