In reply to Bellman:
I couldn't say. I live in Mitcham, which is residential and have had Redwings visit the garden. I have also seen them in local parks and other built up areas, These have been single birds or small groups of no more than a dozen. I wouldn't be surprised if they were there every year, even if it's just a stop off point in London.
Unicum arbustum haud alit duos erithacos
(One bush does not shelter two Robins)
Zenodotus (3rd Century B.C.)
I saw swifts over Battersea Park this morning