Charlie
If you are going to convert a foreign commercial rating to a UK (JAA) commercial it realy does not matter where in the world you get it.
You can take any ICAO commercial and return to the UK, take the ATPL exams (5 months of hard grind at Cabair or 7 at Oxford - the only two residential courses) and take a flight test. Kaboom - done!!
The real question is where are you going to get your first job. The only realy even half guaranteed way of doing it is to become a flight instructor in the US. This is not half as scary as it might first appear, is actually great fun and hones your basic skills in ways that no other job can.
If you train in the US many schools will give you a J1 visa which allows you to STUDY AND WORK for two years. The key is to have as little time of the two year visa on the study and as much as possible on the work. I would recommend coming out to do your private (and instrument if poss) on an M1 visa (study only) and then returning to the UK and applying for the J1 visa. Bear in mind that you cannot apply for a J1 visa if you already have a Commercial from any country. This will give you maximum time to work and maybe even get an ATPL.
The instrument rating is crucial. Most jobs will ask for it later on. The UK instrument costs $40,000 so get an FAA one. Some schools have special dispensation (141) to teach this on lesser requirements so that you can do it post private pre commercial. What a great way to hour build!!! Having an FAA instrument will also reduce the cost of the UK Instrument by a substantial amount.
Helicopter Adventures has been mentioned in these listings and I would recommend them. They are the biggest civillian helicopter school in the world, are extremely professional and the maintenance is the best (which does make graduates a little spoilt on that count). Graduates can be found in most big Helicopter companies around the world which never hurts for later job prospects.
If you are looking for a more rounded education, mountains, fun flying and adventure then I would recommend NAC in Cape Town, South Africa to get your PPL. Cape Town is an incredible city and very cheap. After the PPL go to the US for the rest. The sensible way is to do it all in the US but who needs sensible.
Helicopter Adventures also does a joint JAA/ FAA commercial program - may be worth looking into.
Take everyones figures and double them. Take every time quoted and double it. Take your expected cost of living double it. Add these together and if you have this money and this time prepared then you are well prepared. I have seen too many people have to give up on their dream because they ran out of money. Many of them were pulling down big salaries previously and found it difficult to change spending habits. You are going to be a student which means a crappy car, no restaurants cheap rent and cheap brand foods. This will continue for the first couple of years.
Good luck and let nobody deter you...
And to all you prats out there asking someone if he realy wants to be a helicopter pilot - butt out and let someone live....
SHARK ATTACK