Serin colonised Jersey in the late 70s/ early 80s and grew to about 50 pairs, but for some reason dwindled to extinction by the early 90s.I'd take that with a large pinch of salt as there's been speculation about Penduline Tits colonising Britain for over 30 years but, apart from a lone male nest building in Kent in 1990 (the source of much speculation), it hasn't happened yet. The example of a commoner bird on the near continent, Serin, doesn't give much cause for optimism either since its colonisation of southern Britain has been confidently predicted and eagerly anticipated for even longer but without any real success. With a range of other birds long term breeders just across the Channel, the mere presence of the species there doesn't mean they'll colonise Britain anytime soon. Bluethroat is an interesting case. They've been breeding in wet habitats around Calais and elsewhere along the French Channel coast for decades yet spring records continue to be very unusual just across the Channel in Kent. I also very much doubt that Pallid Harrier will establish itself in the UK any time soon although the odd breeding record can't be ruled out. As for woodpeckers, both Black & Middle-spotted Woodpecker have expanded their range in northern France in recent decades so that they now breed within sight of the Channel. However, with the decline of Lesser-spotted here the prospect of them crossing the Channel seems unlikely.