What happens if you fly from Uk to another country for 3 days and return... will they keep you somewhere? Or can you go home?

upon return you will be in 14 days

mandatory quarantine

Do not tell anybody. But the fact is, if you step outside the UK, even on a cross-channel 'booze cruise' ticket - when you get back the immigration people HAVE to give you two weeks free in a quarantine centre. Which is like Centre Parks only you don't have to pay for it.

Currently, you'll enter a mandatory 14 days quarantine on return to the UK. It can be in your own home.

At this moment in time, you listen to the local rules/regulations ( as each of the countries in the UK are different) and follow what is allowed......... it is likely you will be told to self quarantine for 14 days, but you would need to find out yourself depending on which country you are flying back to and of course which country you chose to go to for 3 days