No, they've been around area the London for a very long time so it's surprising you've only just noticed them. The largest colony is on the edge of Esher Rugby ground of all places where 8,000 plus roost every night!!
Various reasons when and how they arrived - sailors in the 1900's bringing a few over on ships to pet shops in the 1960's and 70's setting them free. Small colonies are now firmly established in the South East and we have a few in Dorset, but as they are highly adaptable and gregarious they could appear just about anywhere. I loathe the noisy creatures as I'm concerned about them competing for food with our own native birds - but most people seem to love them ;-)