Parakeets

I am perhaps stunned by my ignorance, but I had absolutely no idea that there were wild parakeets in England.  So imagine my surprise when I saw, and heard, these big green birds flying around parts of Surrey last week.  How long have they been there?  How prevalent are they?  How have I never seen them before, or even known about their presence?  Some googling was necessary, and the Wikipedia entry about them makes for some interesting reading.  In particular, that they seem to have become more widespread in the mid-90's, and that they are concentrated in suburban London and the Home Counties.  I'm not sure what to make about all this.  I've got to believe they are an invasive species.  And they are certainly pretty loud and pretty noisy, which can't be good for other birds and animals.  And, if I'm going to be honest, I find their presence here somewhat unnatural.  I don't think I would object to a culling.