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.
2 years ago I landed at London City (from Zurich), got off the plane and then we all walked all the way to the exit without being stopped by a single human to check passports or customs. I couldn't believe it.
I am not a British or EU citizen
In 2014 I landed on either Heathrow or another London airport I don’t remember coming from Spain after a vacation
I read on a sign “travellers from Europe this way” and I thought ok my flight came from Spain I’m going that way … when I saw I was out of the airport with no immigration whatsoever
In hindsight it obviously meant if you’re European (which I’m not), I was in shock how easy someone could get in the UK !