Triggerfish (
Balistes capriscus) sightings are far from rare in the UK.
I read somewhere that there is now a resident population under Swanage pier. Unfortunately, I can't find the reference but there are lots of reports of sightings here:
Triggerfish Balistes capriscus (British Marine Life Study Society)
By the way, a lone specimen was caught by commercial fishermen here on the west coast of Sweden a couple of months ago. It had presumably followed with the Gulf Stream from the English Chanel owing to the unusually warm summer and high sea temperautures we enjoyed this year.