No issue just fly in. I am a Brit too. You don’t need to apply in advance. If you want 90 day then yes organise before you leave ( circa £150).
I used to make at least 2 x 60 day a year
Unless it’s changed google advises
You can visit Thailand for a period of 60 days for the purpose of tourism, business engagements, and urgent or ad-hoc work. This visa category can be extended for another period not exceeding 30 days. You can also apply in advance for a multiple-entry visa for up to 60 days.
always take copies as they will send you to the back of the queue again. Passport photo page, passport visa page, printed TM30 and rental agreement for condo is useful