IMHO, the west of the UK is better than the east for diving, in terms of both vis and and marine life. Cornwall (the extreme south-west) has some of the best viz you'll get in the archipelago (not saying much, but it is genuinely better), and some great wildlife (seals, dolphins, basking and blue sharks) and has some good granite reef systems with some rich life. I originally trained on the Isle of Wight, further east, in the English Channel proper, and the conditions there were never as good as they are here. There are fewer complex reef systems, more sediment, and less variety of the life in the eastern channel. I've never dived the east coast at all (North Sea).
In summer, if you don't want to wrestle with a new drysuit (or the leakiness of a rental) you can get away without a drysuit in the south-west, though you'll prob want to limit your dive time a bit. A decent quality semi-dry will see you through, but beware of the wind-chill once you're out - a lot of diving here is from RIBs.