Currently, whether you are fully vaccinated or not, it does not matter, you have to do your pre-departure (PCR or lateral flow) test abroad in order to go back to the UK!

If, like me, you went to visit family over the Xmas period, I am sure it has been a nightmare: tests before, tests when you arrive in France, tests before you leave to go back to the UK, tests after.... without talking about the amount of paper work. During all the process, before and after, at NO time they checked my vaccination status... like they did not care.

So apparently things are changing from the end of this week, as the government just said that Doubly vaccinated passengers will no longer have to take pre-departure Covid tests in England from 04:00 GMT on Friday.
More here: https://www.bbc.co.uk/news/business-59876063