What should I do if my Thai visa application was denied because I'm a UK citizen living in Canada?

How long will you stay in Thailand? You don’t need a visa with a UK passport. You get a stamp on arrival for 30 days and can extend it for another 30 days for 1,900 baht once in Thailand, then you can extend it for another 60 days for another 1,900 baht, and despite what people say, this is seemingly going on for a very long time. It’s been going on since I arrived in April 2021.

If you're in Canada, you need to apply through the Thai embassy in Canada. Alternatively, you could just say you're in the UK and apply through the Thai embassy in the UK.

Where did you apply?

The evisa system did not change any of the rules for applying for a visa, it only changed the method of application. If you are living in Canada, you apply for your visa through a Thai consulate in Canada.

If you are not in the UK you can't apply through the UK, same as before.