Don't worry too much about costs. Thailand is very cheap. I don't know about visa requirements for UK-ers but you can probably get a 3 month tourist visa. It you want to stay long term you may want to consider getting a student visa and studying Thai etc.
In Bangkok you can stay in a luxury hotel for 1200B - or just something basic with air, and no hot water for 400B per night. Longer term accommodation will be around 4000B+ per month for a decent 1 room (+ separate bathroom).
In the country side things will be much cheaper. Renting a room will cost 1000 to 2000B+.
Food will cost you around 30B per meal. Desert as low as 7B. Travel is relatively cheap too. Getting a bus from Bangkok to out to the provinces may cost around 500B for a 4 hr trip.