I don't have any information on accident statistics, but there is more to flight safety than that! For instance is there a difference in airspace busts, people getting lost etc. Not just how many people hurt/killed themselves.
I have re-trained many people who have come back from the states. In many cases, it has often been just that, an almost complete work up. There are good schools and bad schools wherever you go, but on average and taking into account the environmental differences the U.K schools produce a higher quality product for our conditions. There is more to flying skill than just how you manipulate the controls and how good your landings are.
Personally, I'm not too worried about people going to the States from a business point of view, the club I'm at is non-profit making and I make my living elsewhere so we don't suffer from the same issues as most schools.
Weatherwise there are better places to learn the basics of flying than the U.K, but once you get past the early general handling part of the syllabus and you start to be aware of what is happening around you, that is when you should start your flying in the U.K. For example, some of the big commercial schools (Oxford especially) used to send their students abroad for the initial part of their flight training as it was cheaper, but then they brought them back to the U.K for the more advanced stuff. Why do that if you can do all the training in the U.S? (excepting the IR for obvious reasons.)
Actually, extremes of weather are often easier to operate in than the normal British muck.
Why? If you see that the wind is blowing 50kts and there is a hurricane or thunderstorm approaching, then the decision not to fly is an easy one, but here in the U.K we don't often get those extremes so making the decision to go/not to go is more difficult and people being people the old rules about 'if there's any doubt, then there's no doubt...' go out of the window unfortunately.
Push-on-itis is still a big killer and that is usually because the pilot concerned didn't read the weather well enough before flight and then compounded the folly by making bad decisions in-flight. IMHO someone who has been subjected to the vagaries of the British weather and is used to cancelling flights due to the conditions, is less likely to launch off into dodgy weather than someone who is more used to going flying every time they arrive at the aircraft. It's easier to make a tough decision if you've got a standard to work from i.e knowing when your instructor used to call time on flying gives you something to go on.
I like to take students up in not very nice conditions to give them a small taste of why you don't do it. A difficult thing to do if you are on an intensive course in gin clear weather.