To expand a bit on what Whirls is getting at......Flight schools are first and foremost profit making entities (well the properly run ones anyway). The ones in the US tend to offer a package price as illustrated in the various flight magazines. This generally works out much cheaper for you and of course they make a fistful of dollars as well. If you start wanting to trim down the package by virtue of having say 20 hours with another outfit you will stumble across a number of obstacles. Firstly, they will probably say no outright. If pushed they will then give you the whole transfer of training records argument. Further, they will in no way shape or form be willing to get on the phone and make international calls to arrange for delivery of these. Finally flight schools are inherently biased against each other. Each one thinks it is the be all and end all of flight training and any hours you did somewhere else will be scoffed at in anycase.
I know that in the UK there are a few providers who allow you to sit the PPL exams in advance of the training. That can help out considerably if you are planning on heading out to the US to do a JAA PPL. It will lift a major weight off your shoulders having all the exams in the bag prior to departing and that way you can focus exclusively on the flying side of things. Do a pprune search to find out who offers this and check the private flying forums for info. They should be in LASORS as well which you should download from the CAA website as it contains an up to date listing of all the UK CAA approved JAA training providers.