It could have helped the UK, but you have to make use of it.
If "it's much easier to control population flow into an island" then you still have to step up and control it. The UK government did not even show interest in doing that.
The UK government mismanaged their borders, and most other aspects of the pandemic; notable exception being vaccination.
The UK gave up the ability to do that when previous governments decided to integrate our economy with Europe and build the Channel Tunnel. Every day, huge numbers of trucks travel directly from Europe to the UK with their drivers carrying all kinds of goods, including critical stuff like food for supermarket shelves. You may recall that this was one of the reasons why Brexit was supposed to be so dangerous and impractical.
There are many things that the UK government could have done at borders that they did not do, or delayed and did only when damage had already been done. It is a mischaracterisation to suggest that it was out of their hands, or that the UK faced unique problems that other countries did not also have, and handle better.