Teajunky and Nigellas the p+p from Alldaychemist is an awful lot (and more than you pay for the product itself) it's true - but I'm prepared to suck that up for such cheap products.
As for whether retin-a is genuine or not if you buy it online - honestly this stuff costs peanuts to produce (the mark-up they're making on it charging the prices they do in the UK and USA is obscene) and it would not be cost effective for anyone to try to fake it. Any retin-a or generic equivalents I've bought off the internet have always been the real deal.
As for whether it's now available in the UK, even on script, I did read that some of the products had been discontinued and that certain strengths weren't available anymore (the 0.5 cream for example). Whether that now applies across the board, I don't know.
Lastly I just wanted to add that anyone on this thread thinking of trying it, please be careful. Retin-A is a drug, not a cosmetic. The reason you need a prescription in many countries is because a) it works and has therefore been classified a medecine, and b) because the side effects can be serious if you don't use it correctly. Follow the instructions. Use a small amount once every three days or so and build up slowly from there. Don't use it if pregnant, breastfeeding, or trying to concieve. Also this stuff makes you far more sensitive to UV, so you have to use sunscreen every single day, rain or shine.