What are the rules for UK tourists regarding visa exemptions and entry limits to Thailand?

Although there’s no offical limits other than the 2 land borders, be wary about entering via the 2 major airports in particular if you have spent over 180 days in country in the past 12 months. The immigration officers at the Bangkok airports in particular are at their discretion pulling aside people who have been in country over 180 days as a regular tourist. It happened to me on 1st May and I needed approval by their superior to enter. If you are over that limit then you really need to look at other options to extending a visit ie making use of the 2 land border bounces (with a border run company) if not yet used, or applying for METV well in advance of a long planned trip.