I’ve done courses both with BSAC and PADI, and I wouldn’t rate one over the other. I’ve been a member of an utterly useless BSAC club where it was nigh on impossible to make any progress with your training, and I’ve also been part of a brilliant club who couldn’t do enough for you. I’ve heard horror stories of PADI instructors, but all the ones I’ve had experience of have been highly competent, patient, and keen to make sure their students were of a high enough standard before signing them off.
As for learning in the UK, all my training dives have been done over here, and my first taste of open water was Capernwray in the middle of winter. Yes, that would probably put a lot of people off, but I loved it. I finished my BSAC Ocean Diver course in Vivian quarry, Llanberis with a leaking drysuit in March. It certainly makes you appreciate warmer water diving that bit more! But at if what you want is to learn to dive, combined with a week somewhere warm, go for it. My choice in that case would be do a referral course to get the theory and pool work done before you head out, finish the Open Water course in warm water, and then if you wanted to give UK diving a go further down the line, do Advanced Open Water with a centre over here. Or find a local BSAC club, and do a cross-over course with them.