Hiya,
I also faced the same dilemma. In the end I brought some of my gear and bought some over here. But I got a travel BCD which folds up small which is ideal for backpacking. Also, slip on fins and a shorty wetsuit - ideal for warm water diving which the UK is so close to!! If it gets to cold just hire a suit or other gear required where ever you are!! Definitely bring your kit as it can be expensive to hire!!!
Most of the UK inland dive sites (IN MY OPINION) I wouldn't waste my time with. The vis is usually crap, they are cold, dark and nothing in them but old cars etc. One good thing is that they are used for OW training and if you can dive in them you can probably dive anywhere!!!
But the coastal dive sites are very rewarding!! Scapa Flow in the Orkney Islands (Where you say??? Off the top of Scotland) has probably the best wreck sites in the world!! There are also so many other UK dive sites that offer many things to see. But be realistic - it isn't going to be coral reefs like the Red Sea or The Great Barrier Reef - it is just different types of diving. It is what you make of it!!!
But the main advantage of living in London is the ability to pack your bag and go diving all over Europe and the Middle East - flights and packages are quite reasonable and the diving can be very diverse from cold water in Iceland to warm water in the Red Sea!!
Another London club is
www.urbanfish.co.uk
My pic of places so far not in any order is:
UK - Scapa Flow - AWESOME but challenging!!
UK - Cornwall - good vis and a lot of wrecks and history of the area!!
Ireland - West Coast has lots of good places!!
Egypt - Bells & Blue Hole in Dahab, Hammerheads in Tiran, Ras Mohammed from Sharm El Sheik and not forgetting the Thistlegorm wreck!
Egypt - liveaboard - the only way to see it all!!
Malta & Gozo - caves and wrecks, good food and sun!! Well worth it!!!!!
Tenerife - good boat diving.
Sardinia - great Med diving
Greece & Turkey - great Med diving and snorkelling
Am planning on a lot more!!!! Any ideas welcome!!!!!
Good luck!!
Dougz