It didn’t.
The main reason old photos look to attractive, and the streets look so clean be because of the image quality — the gum trodden into the pavement doesn’t show up, nor does the grime in the lampposts. In fact, due to poorer hygiene, the UK being a poorer place then, and pollution (many buildings still had soot on them), the UK was probably dirtier.
Looks clean and colourful (Carnaby Street 1960s), but upon closer inspection is actually grimy and tacky (cheap-looking by today’s standards).
Compare that to now. If you look closely, it’s exactly the same location, but some buildings have been painted.
This difference is even more pronounced when it comes to older black and white photos, where detail and colour are all but invisible.