Originally Posted by Oldtiger

I assume that you have a UK passport and, therefore, qualify for a Visa Exempt entry into the Kingdom thus giving you permission to stay for 30 days. If this is correct, you don't need to leave the Kingdom for a few days and then return to obtain a Visa Exempt entry for another 30 days. When you are in the Kingdom you can go to the local immigration office and apply for an extension (30 days) at the cost of 1,900 THB - much cheaper than cost of a return flight to a neighbouring country.
The only proviso to this information is that you should not have a long history of back-to-back Visa Exempt entries as immigration may think that you are not a tourist.