Is it acceptable for UK passport holders on a visa exempt to fly out and back in a few days for a visa run?

Stay out more than a couple days. Maybe 5-7

Border bounce or visa run?

As long as you are following a pattern of being an actual tourist, as lots of people base out of Bangkok, and other parts of Thailand, then visit the surrounding countries, spending more than just a few days then coming back. What really sets off red flags if it is always to the same, closest, country, but if you spend a normal tourist amount of time out of Thailand, then coming back, you're fine for quite a long time, as there are countless tourist locations within a few hours flying from Thailand. It's even cheap for couples to do. When you are following a pattern of being a tourist immigration doesn't bother you. You'll be fine, just spend a few more days away seeing the sites of other countries close by.

visa run with no visa and a visa extension with no visa. Complete wrong question.

If you don't have a history of staying in Thailand you should be OK but probably won't be able to do it again by air. It's best to use a border run agency who will transport you to the border and ensure you can return

You doing an actual visa run or a border run ?