If you have been in or around London, you will know the ones. They aren't native, and there's been theories as to how they got into the wild. One was they originally escaped from filming at a studio in the 1950s. There's now been anecdotal evidence of this.
Hana Ayoob, a curator at The Cheltenham Science Festival, said her great grandfather had lived near to Worton Hall Studios. He recalled that after filming finished in the early 1950s, "little green parrots" began to arrive in local gardens.
I expect just like Pheasant,some rich knobhead decided it would be a good idea to bring them here and let them go.I think.it would have to have been after 1850 because before then the winters would have been too cold for them to survive or stay.