hey paolow,
i am also british and am planning to hike the PCT next year with my girlfriend who is from Slovakia.
i am noy sure how other europeans have managed an extended stay in the usa but i am aware that a normal 3month tourist visa is insufficient and that we will have to make an application to the US embassy in london to get an extended stay visa.
this procedure is not as simple as you might think as you have to be able to prove to an embassy official during a 1 to 1 interview that you have no intention of staying longer than the term of your visa.!
if you have any questions about this feel free to send me a mail and i will give you telephone numbers and websites that might be helpfull to you.
i personally see this extended visa situation as the major stumbling block asa to attempting a through hike.
i know other british people have completed through hikes in past years but have never been in contact with them.
nelson66