Always send in £. Your bank will default to baht so make sure you change it. You’ll get a much better rate if the Thai bank changes the currency. There will be a fee but it isn’t very high.
You can get 30 days Visa Exempt entry when you arrive and extend that for another 30 days for a cost of 1900 baht BUT the airline might insist on seeing a ticket out of Thailand within 30 days when you check-in.
Personally, I’d get the Tourist Visa before travelling which gives you 60 days, especially if you live near the Embassy or a Consulate.