You could get a single entry tourist visa in the uk. That will give you 60 days in Thailand which you can extend here to get another 30days. Uk passport holders cannot get a visa on arrival, only a 30 day visa exempt stamp at immigration.
Application form and 2 photos. Plus photocopy of passport, which you can get the relevant pages done inside the consulate for 20 Baht. Don't use the touts outside. As they don't want proof of address you get just put any hotel down on the form.