Can I apply for an e-visa before leaving the UK and activate it after entering Thailand on a visa exemption?

Here's the Official Royal Thai Embassy Portal link.

You can apply here and get your visa within 15 working days. Good luck

Three 30 day exempt two extensions will get you five months. But you'll be done for a year.

Yu have a current EVisa or has it expired? You can re-enter on a30dY visa exempt

Yes you’ll have to leave and stamp back in on your e visa

Multi entry tourist visa.

60 days in, 30 days extension, go out of country, come back and get 60 more days before another 30 day extension.

If your timing is right, and with the 30 day extensions you can get, and that your evisa will be good for 90 days. After your first 60 days from the visa exempt 30 day entry stamp, and the 30 day extension for another bt1900, and the cost of getting to immigration, you still have time before your evisa expires to just leave and come back for another 60 days, and another bt1900 30 day extension, and you are almost at 5 months. You could cross and come back for another 60 days, or until your flight home, but don't try to push it past that without touring around other countries, or they can determine you're trying to stay long term as a tourist, and will tell you to get the correct visa, and refuse you entry. At that point is usually requires another tourist visa from an embassy, and then flying back instead of land border crossings.

You can apply for a 60 day tourist visa from UK. You don't get VOA, you get 30 day visa exemption at arrival in Thailand

You don't get a VOA, you get a 30 day free entry stamp. You then have to leave the country and re-enter to activate the tourist visa