Anybody can get in anyway.
The UK has a few thousand miles of coastline, and you cannot stop a boat getting in, especially after dark.
This is the reality of the "free world".
The country I come from had dual barbed wire fences, about 30m apart, all around it. The fence was electrified, lethally of course. The space between the fences was mostly mined with anti-personnel mines. There was a road all the way along, patrolled regularly by armed guards.
That worked pretty well; short of Special Forces know-how, somebody getting you a foreign passport, or you brown-nosing and creeping around the Communist Party circles for many years until you were "trusted", you would not get out (or in, though of course nobody would want to do that anyway). Even lakes which were shared with the "decadent West" were closely patrolled, with underwater nets, although that is definitely where I would have crossed had I still been there.
The only way large numbers can come into the UK is via conventional overt routes and those are all controlled.
Border controls are meaningless for controlling individual mobility.
All that limits mobility nowadays is controls on employment (illegal work offers pretty limited options, unless you are female and desperate) and the fact that not everybody wants to uproot everything and move to a country where the tabloids, and many people, will hate you and where everything costs a fortune.