As I know
https://cex.io is UK based exchange. At least that's what it said to me on the bank statement when I made a withdrawal from there.
Damn, I completely forgot about them and am a bit surprised they're still around.
They use to be big when they were selling mining contracts and their pool (ghash.io) at one point even crossed 50% of the global hashing power. But they seemed to be slowly dying after mining contracts become unprofitable.
It's hard to figure out what would "based in UK" even mean nowadays, they seem to be run by multiple entities and the only regulated one would be a Gibraltar-registered company.