Can I book a one way flight to Thailand with a UK passport, and do I need a visa?

The airline from the UK to Thailand might be the only issue.

You can also get an "onwardticket.com" very cheap and it will take care of the 1 in a 1000 chance of being asked. Many similar sites.

I would get a cheap onward ticket as I am normally asked to show one at the Airport when checking in.

In 18 years, I've been asked to produce an onward ticket once. Recently, I've been asked if I have one ever time, but not asked to produce it. I have loads of 30 day stamps where I stayed for three weeks or less, then returned home. I'd get a cheap onward flight (not a bus ticket) just to cover yourself, because I have seen people told to buy air tickets or no entry, unless they have a Visa.

Its the airline, i fly a lot into Thailand before covid would be asked maybe 5 % of the time to show visa or onward ticket. Since covid i reckon its about 50 % of time get asked

you might be in for a surprise when checking into a flight with a one way ticket and no visa. And find out the airline doesn't give a royal fart if you show them a fake onwardticket or a bus ticket to the border which the Thai bus will actually not cross! Be careful with advice of this sort in Facebook. Go buy a REAL cheap one-way flight ticket out of Thailand within the 30 days you will get stamped in upon a visa-exempt entry. For example Had Yai to Kuala Lumpur with Air Asia for 35 bucks (dollars!) and use it or throw it away

Book a flight and fly to Thailand where you will be given 30 days visa exempt stamp. If, in the massively unlikely event that the airline wont let you board without onward travel proof, have the ability to book some cheap bus service out of the country, on your phone. No need to buy anything extra unless they insist ( which they won't).

I would definitely buy a onward ticket to another country like Cambodia or Vietnam to be safe. Get a cheap one as we call this a throwaway. If you want to stay longer, do a boarder run, it will give you another 30 days

You can get a 30 day Visa Exempt entry and a 30 day extension but it's very possible that your airline will require proof of onward travel within 30 days of arrival. You can ask them their policy and Google "onward travel ticket" to satisfy the requirements if required.