There are other solutions in this thread which might answer your questions.
I haven't checked ebay lately. But a couple of months ago, I got a three sim card with no credit for 99p (about $1.22) and this particular vendor ships to the USA for 99p or at least did at that time. Go back through this thread for that link.
Now the idiots at 3, like many of the other British telcoms do not allow topping up with credit cards issued outside the UK nor do they take paypal for that. That was also discussed here. However, there is a web site,
www.mobiletopup.co.uk that will allow you to top up UK sim cards online and will take paypal for a 99p surcharge. They email you the top up code and after you insert sim card and the phone registers on a US roaming partner (either T Mobile or AT&T) you follow the directions and can top up and have a fully functional phone as soon as you arrive in the UK. or
You can go to the three web site and, again at least they have in the past have them post to your hotel address 1 or 2 sim cards for free. Upon arrival, you insert the sim cards then go out to either a grocery chain, a service station or a drug chain (Boots or Superdrug for example) and tell them you want a top up. They will process it at the register, absolutely honor your US credit card, and then you enter the top up code as above. All this is free. or
You can go into a phone store, say Carphone Warehouse, upon arrival in the UK and they'll take care of all the details. The sim card is free with a minimum £10 top up.
All these methods will work fine. The advantages of doing it here for the small surcharges is you know your number in advance in case you want friends and relatives to have it and you have a fully functioning UK mobile phone as soon as the plane hits the runway at Heathrow or Gatwick (nobody pays any attention to the old rules to keep mobile phones off until inside the terminal although in the USA they've given that up and now tell you on flights you can turn on your cell phones while the plane is still taxing to the gate don't know about the UK airlines).
Hope that helps.
PS...Here's the link for the 3 sim card.
http://www.ebay.com/itm/UK-Three-UK-...oAAOSwP~tW3qFf
The postage to the US is only 75p not what I gave you.