London has a massive population of feral parakeets, they can survive quite far north of the weather is mild. London is basically an urban forest so that does help.
I have lots of them showing up outside my office window... E.g. [1]
But the occasional survival eventually leading to a breeding population doesn't mean the odds of survival for released/escaped birds who have grown up in captivity isn't really low.
Yeah, the explosion in numbers has been rapid, and they hang out in groups and can be real bullies to other birds so they'll definitely have an impact.
On the upside, they're also increasingly supplementing the food supply for peregrine falcons in London (they're apparently easier targets than the pigeons...
The parakeets have reached Birmingham as well. Often seen in Canon Hill park, Highbury park or Yardley Old Village around the church. Always brightens my day.
https://en.wikipedia.org/wiki/Feral_parakeets_in_Great_Brita...