But I find this hard to believe, why would any country let you in without recent proof of being covid free?
If you have had both vaccinations you can travel around the majority of mainland Europe without any need for proof of a negative test. I am in France and the pharmacies have a steady stream of people queuing for their tests to go back to the UK, but no (AFAIK) other EU countries require this.
In order to fly back to uk, you have to prove you have a PCR booked for when you get home right. On the passenger locator form to
Get back into the uk, it's not the day 2 antigen test they ask for
Is it? So why have that done? To whom do you report that you've had a day 2 test done.
You don't have to "report" it to anyone. It is simply a money making exercise. Either pay for the test or you cannot get on a flight/ferry to the UK. You could not do the tests and nothing would happen. But you sure as hell have to pay for them and prove that you have done so.
So why the antigen test when you are there. What's the need for that? Who asks you to prove you've had it done?
You need proof of a negative test to board your flight.
The logic behind much of the above is nonsense - you could have a negative test 24 hours before you fly, but all that means it that you didn't have Covid at the time you were tested
Presumably the day2 once you return is to check whether you caught Covid from anyone on the flight but seeing as nobody checks whether you actually do the test then it is simply a way to force people to buy a testing kit (regardless of whether you use it).
The simple answer to all the why questions is that "themz the roolz ".those are the current regulations.
On a more positive note, there was an article on the BBC news page yesterday suggesting that rules may change:
www.bbc.com/news/business-58491245
Fingers crossed that this is the case as we are booked to go to the UK soon to meet our new grand-daughter whose arrival is imminent 
I hope your trip goes smoothly @Oblomov21