Don't worry about cold water, just get a dry suit you can wear your thermals in and you will be nice and toasty! The UK has quite a few caverns you can dive in, pretty cool stuff, I dove Chac Mool in the Riviera Maya (just south of Playa del Carmen) last month, and it was an amazing dive! It is what kicked me in the ass to continue my training.
Where I live there isn't anything great to dive, though the quarry I just dove yesterday was actually pretty fun, only about 10 m of visibility and a depth of about 17 m, but it was a good time. I live right on Lake Erie, one of the Great Lakes, but the visibility sucks. However, there are literally hundreds of shipwrecks in the great lakes so I look forward to starting some wreck diving, but probably not till spring now as it is getting late in the year (cold!), and I don't have a dry suit
I got my open water cert on the Great Barrier Reef when I went to Australia for a month backpacking solo about 2 years ago. I have dove off of Negril, Jamaica, and several times off of the coast of Cancun and the Riviera Maya, as well as the 2 cave dives in the same area. Also some local quarry diving.
My plan is to get my advanced and nitrox certs this next month, get my first aid and CPR certs this winter, then get my Rescue Diver cert in the spring. From there I will go for Master Diver and then the first PADI professional certification, Dive Master. I hope to work at the local dive shop on weekends as a dive master as part of my training. That is my current goal, though I may extend it then to an assistant instructor / instructor.
Not only do I want to be a better diver, I would like the possibility to have the experience to get a job as a divemaster or instructor somewhere else in the world, possibly Mexico, and eventually move down there and work. Or perhaps Thailand or the Phillipines. Somewhere where I can make enough money to support my long time girlfriend so she won't have to work and we can live the pure life.