Regarding the license validation it's covered in a few places here. The only relatively new thing is a form that you have to send to the CAA so they will send a letter to the FAA to confirm your legit, costs you of course, think around 30 pounds. There's also a form you send to the FAA which you can get off their website. You have to nominate an FSDO office in the States to collect your validation from, there's one at Mongomery Field in San Diego. Once things are matched up in the States you get a letter from them. Then you make an appt to go in and get it sorted. Takes 20min. Everything from the FAA side is free, and you eventually get a shiny credit card style licence in the post (you get a temp one in the FSDO office thats good for 120 days until the plastic arrives). I'd advise you to do this all well in advance, they say it can take up to 90 days to process but with me it was much quicker. But still better to be safe unless you want to pay for an instructor to fly everywhere with you.
Hope that helps