For gsm phone, the best two options are TMO and AT&T prepaid. Either get their monthly plan or their $2.00/day plan [unlimited for voice/txt/data]. If you go to their respective websites, more details. I think monthly unlimited plan runs less than $50/month.

As far as getting it organized ahead of time, amazon usa store sells the prepaid sim cards here: http://www.amazon.com/s/ref=nb_sb_no...epaid+sim+card. Check if amazon uk offers the same. You might be able to get the two carriers to ship to you but I have no experience with this.

If you have a friend stateside, he/she could purchase and activate, and send to you all ready to turn on upon arrival.