I'm not sure this qualifies as a rarity, but walking on the Thames path at Runnymead I saw two Rose-ringed parakeets. Could they have escaped from somebody's garden? I'm sure tha the locals around Windsor must keep some interesting birds in their private collections.
bout 25,000 rose ringed parakeets in london and the surrounds so i think they were probably wild(ish) ones
Hundreds of them here too.