UK, sorry I mean the British Isles...
Scapa Flow in Orkney, Scotland, is great diving where a lot of dive boats operate. It's relatively shallow with a max of 45m/150ft and is sheltered so you won't loose too many days if the weather's bad. It isn't easy to get to as it's at the back of beyond and three days past that on a yak. The diving and the history of the diving is great though. Generally considered to be a should do dive.
Malin Head in north west Ireland is utterly amazing diving, probably the best around the British Isles. There's big wrecks at 70m/230ft MOD2+ depths and deeper at MOD3 depths which are simply sublime. There's a couple of boats which go out of Malin both are well set up and supply gas. You will need to arrange for rentals which can be done through people who organise trips up there (Kieran Hatton
divingindepth.co.uk who took these photos). I've been a couple of times and it's easily the best technical diving I've done. Weather's an issue though. We managed five of the six days but made a trip up to the Giant's Causeway for the off day (highly recommended).
Submarine at 70m/230ft:
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The "Dutch Titanic", the Justicia:
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Laurentic at 40m/130ft:
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Look closely at the Sherman tanks on the Empire Heritage:
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