Yes. It's not a visa its a 60 days visa exemption for British passport holders but you do need to fill in a digital arrival card before you arrive, but no earlier than 3 days. :)

Believe what you want my friend but met my VIP rep at D5 which was pretty much next ro my arrival gate, stamped in 3-4 mins later at immigration (no line - same place as aircrew go) and with hand luggage only the car was waiting for me outside. Cost me B2900 including a taxi to the hotel. Only way I arrive Bangkok now and have done so every rime in the last 2 years (6 times).

I use every time. 6 minutes last week from plane to taxi !

It will class as an overstay I believe and you'll get a fine as well as having trouble getting back into Thailand in the future.

Personally I would fly out of the country on a cheap flight before your 90 days expire and then come back on a connecting flight to meet your original flight.

You could try and fly out and back again next day and get another 60 day visa exemption but you risk a refusal or questions asked at immigration.