Can I extend my stay in Thailand for 30 days with a UK passport upon arrival?

Yes, you can get 30 days extension, see here my experience in Samui

Mark. I’ve just come back to uk. Been for 75 days. 2 nd day there I got my 30 days added. Was very easy. Print form off the internet. Fill in. 2 passport pics. Passport. Evidence of residency. 1900 baht . Was in and out in 45 mins. Passport stamped !

That was phuket. May be different in other offices? Just sharing my experience 👍 best of luck.

Yes, you can do it online, quite easy

going to the immigration is a pain in the ass and costly, either do an e-visa or if you will stay close to a land border cross to a neighbouring country at the end of your visa and come back to get 30 days again.

You don’t go to the embassy you apply online, should only take 3 days to receive it by email and you print it and keep it with your passport.

You can either arrive with no visa and get stamped in for 30 days, and then go to the immigration office once you're here and apply for a 30 day extension.

Or you can apply for a tourist visa through the evisa system. The London embassy is known to be very fast and you should receive your visa in email within 2-3 business days after applying. Then you won't have to worry about going to the immigration office while you're here.