Rupert,
I did my PPL in the US. The main advantage is that if you set out to get your PPL in 4 weeks and choose somewhere with good weather, you can do it. In the UK if you are unlucky with weather it can take much longer. The main disadvantage is that when you get back you have to get used to UK airspace, radio, etc which takes a little while.
On the cost side, I'm not sure if there are still cost savings to be made so when you are doing the comparison make sure to add in the extras like accomodation, books, exam fees, visas, and conversion / signoff costs (see below)...
In some places in the US you get a CAA (UK) PPL but there is less demand so fewer schools to choose from.
If you get an FAA PPL you have many more choices of where to learn. Currently you can fly in the UK on an FAA PPL. You'll need a few hours of dual time to rent privately so take this into account when calculating your costs. If you plan to fly on your FAA PPL you will also need to get a radio licence: in the US there isn't any separate radio licence (it's part of the PPL) but in the UK you need a separate document. You can get a standalone UK radio licence or get a document issued by the (US) FCC. (Both will involve extra costs.) When the EASA (EU) rules come into place I don't think it will be possible to fly on an FAA PPL any more (I don't claim to be expert on this so others on the forum should correct me if needed).
If you get an FAA PPL and plan to convert, there are different processes depending on how many hours you have. If you convert with less than 100 hours of flying time, you will need to sit all 7 UK PPL theory exams as well as do the checkride. If you have more than 100 hours, you only need to sit 3 exams and do the checkride.
I hope that helps.