No experience with UK, but I am fairly certain they have refill or top up cards that can be widely purchased. Based on the German & Malta prepaids, I am going to guess that if you want to refill online, you would need to register your card/number online. You may be able to top up by using your credit card at ATMs or other places, but I cannot guarantee that; is possibile in Germany also at the post office.
Where were you going to buy such a card, here (Ebay) or there (after you arrive)?
Why don't you check on Ebay and ask your questions?
Here is an example:
http://cgi.ebay.com/NEW-ORANGE-UK-PAY-AS-Y...1QQcmdZViewItem
Not saying that offer is good or recommending it an any way.
Stan