I hold a UK PPL, night IMC, all done in the UK and it took about 18 months in total.
I also hold an FAA standalone PPL, IR and hopefully soon a CPL, all done in the UK except the IR which was done in the USA.
And I've done the Visa/TSA process, have even written up detailed notes on the whole thing (for the benefit of other pilots), so I do know a bit about it...
People who try to compare are never going to compare like for like.
IMHO it is almost irrelevant whether you train in the USA or the UK. What matters is whether and how you can fit the training into your life.
If you go to the US then you will be living eating and breating aviation for four solid weeks. Unless you are sub-capable, you
will do it in that time and you will reach a good standard of basic flying. In Arizona the weather is guaranteed (but you can't do a JAA PPL there; the nearest is southern California) so not much chance of a cancellation. Florida isn't great but they usually work around the afternoon weather.
If you do it in the UK by checking into a B&B for four weeks then you will also do it in four weeks - weather permitting. Realistically, unless you pick the Autumn months, it will take 6 weeks. But that assumes a well organised school; most UK schools don't have the organisation of the big US ones.
But almost nobody does this - people choose the UK option
because they can slot the training into their lifestyle and/or income stream. And they take a year as a result, spending probably an average of £8000.
I don't think there is any difference in the end result. From the USA, you come out a good pilot and you have to learn a bit of UK radio calls. From the UK, you should also come out a good pilot but you have reached this point a year later and with about £3000 extra spent. OTOH, being able to fit training into one's life/job can be priceless, which is why I have done some of my FAA training in the UK, even though it is a massive hassle and a huge expense.
There are stories of US trained pilots who came back not knowing that in Europe they need an explicit clearance through Class D, etc. This level of dumbness is just about possible but you will find it everywhere. The other day I heard an ATCO patiently explaining to somebody (G-reg) which direction EAST is in; the pilot kept turning WEST towards some CAS.
When my 11 year old son is old enough to log training (14) I will find him an FAA CFI to log all the possible hours, and at 16 he can formalise it by going to the States and flying the remaining mandatory hours over a couple of weeks. This will be a rather extreme example of somebody with (by then) hundreds of hours of unlogged time, and there would be no point in hanging around a UK school in UK weather.
Much is made of the need to fly in UK weather. For goodness sake, a PPL is supposed to fly under
VFR 
You aren't supposed to fly in cloud, in the UK, in the USA, or anywhere else. If you can't plan a flight so as to remain VMC, you have to scrap it. That's VFR... and yes it's rather limiting. VMC in the UK is every bit as good as VMC in Arizona.
So, I don't think there is a relevant difference in the standard. It's horses for courses.
The biggest challenge seems to be hanging in there post-PPL. Most don't - all but one of the people I met when training 7 years ago have (
AFAIK) given up long ago, and most disappeared almost immediately. But that's another story.