What should I do after my 60-day tourist visa in Thailand expires?

If you check your entry stamp in the passport you will see the date you must leave by or purchase an extension

@Lynda *******

so if you got a 60-days stay permit until November 30, you can extend it on Immigration for 1900.- THB for 30 more days, carrying you until 30 December. Then, BEFORE the extension expires on December 30, you will need to exit Thailand. Thus, let's say, if you stay abroad for 9 days, and return to Thailand without a visa, on January 8th, you will get stamped in visa-exempt for 60 days until March 7th. You will need another 30-days extension to get carried until your return flight on March 31st . . . . applying for another 60-days single entry tourist visa on a Thai Embassy/Consulate abroad, won't cut the ice, it would actually result in the same amount of days admitted stay permit

As a UK citizen you did not need a visa for 60 days.

You will exit as your extension expires and reenter with just a tourist visa exempt stamp for 69 days.

Which you can then extend for 30 days.

Check the date on the stamp in your passport. TR visa is 60 days.

You need to check your entry stamp in your passport, while you might have a '90 day visa', this only means it has a life of 90 days ("use by"), on a tourist visa they would only stamp you in (allowed to stay) for 60 days.

What visa did you buy from the embassy before you traveled to Thailand? If you indeed had a 90 day visa then you most likely had a non-O visa and there is no 30 day extension available for that. The 30 day extension is only available if you're on a tourist entry which means tourist visa or visa exempt entry.

A non-O visa can only get a 1 year extension after meeting all the requirements.

The tourist visa is for 60 days not 90. What visa are you on?