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Well some old pokemon should be native to the UK region, maybe with some regional variant to their appearance. Snubble line, Lillipup line etc. Maybe Honedge line (King Arthur-ish inspiration)
For new pokes:
Badger, Fox, Red Squirrel, Weasel, Vole/dormouse, English Robin, Mountain Hare, Scottish Wildcat, Kestrel, Puffin, Wildcat, Golden Eagle, Slow Worm (It's actually a legless lizard, could play with that gimmick), Blue Tit.
Then there's plenty of fictional folklore stuff for legendaries. Welsh Dragon, Loch Ness Monster, Fairy queen (They originated in Ireland), Hound of Baskerville/black dog/devil hound
