These little green parakeets appeared in South London some time in the 1990s and there is quite a flock of them these days. Their origin is not precisely known (from Wikipedia):
"They most likely came from a single pair of breeding parakeets which escaped or were released in the mid-1990s. Other origins have been attributed to them: the most popular idea being that they escaped from Ealing Studios, West London, during the filming of The African Queen (which was actually made in the Isleworth Studios) in 1951. Other theories are that they escaped from an aviary during a 1987 hurricane; or that a pair released by Jimi Hendrix in Carnaby Street, London, in the 1960s, are to blame."
I noticed that in spring the parakeets feed on buds and flowers.