I think you can now do the lasergrade testing at Flightsafety EGLF. Another possibility is 'Business Air' at Norwich. I tried 'Soloflight' a few months back and they told me that they have not been able to provide FAA computer testing for some time now.
Best way I found for passing the tests is to get the 'Gleim' cd rom and practice doing the real questions on your PC until you can regularly score above 90%. The FAA exams are fairly straight forward compared to the JAR groundschool but the oral part of the flight test can be quite daunting and will find any holes in your knowledge should you fail to prepare.
TSA and Visa application as mentioned above is a little bit time consuming but when you chose a school in the States they should be able to guide you through the process without too much pain.