This is because the UK doesn't have exit checks. They rely on airlines to submit the information to them.
I guess this makes sense when you consider that there's an open border with Ireland. Though you'd think that the UK and Ireland could get together to track exits...
20+ years of lighting our hair on fire over immigration and we still have no idea who is in the country.
> I guess this makes sense when you consider that there's an open border with Ireland.
Weren't the other borders with the Schengen area open, too? Eg if you take a small boat from England to Denmark, no one needed to check anything.
In the context of the issue that doesn't really make sense. The issue is that the home office think you left and didn't come back. How would an exit check tell the home office you have come back into the country?
The UK's borders used to be hilariously lax. In 2000 I travelled a lot. To leave, as you note, you just left.
To return, you'd walk past a man at Heathrow who was invariably reading the paper. He had his feet up on the desk. You were walking at a clip, passport held aloft, photo page ostensibly open towards him.
That was it. Immigrated.