Just did my FAA>JAA CPL conversion, took me a little over two hours plus the flight test. But that's because I know the plane involved pretty much by heart... And yes, I did the training and test entirely in UK airspace, following the IR conversion. Didn't fly in the UK before my conversion.
I'd suggest you either go through a full and structured professional pilot program in the U.S., followed by a conversion, or a full program in Europe (both options will probably end up around the same price, no matter how you twist it). That's looked at more favorably by the airlines than picking up you licenses and ratings here and there.