Ring necked parakeets were introduced to London in the 1950's probably from escaped pets though other colourful stories include being released from the set of the African Queen film and Jimi Hendrix letting a pair go.
They have adapted to city life quite well and now found all over london though Hyde Park is great place to get near the parakeets.
They squabble with each other for food and only seem tolerant of there partner so when another bird outside of there social circle lands they can get agressive. In certain parts of the park they will eat from your hand, be warned they have sharp talons and can bite.
One of the concerns is that they outcompete native birds for food and weren't happy when the local pigeons tried to get a feed! They also take over next nest holes that woodpeckers and other birds would normally use. London is the strong hold for them but have been found as far north as Glasgow with patchy populations all over Europe.