Speaking as an ex-T-Mobile UK employee...
When you see the 'E' on your iPhone it means that the phone has detected the network and is able to use it. When you try to use the Internet, it fails because you have the wrong APN information is, as you said. I see that you later said that you'd put the correct APN details for Virgin in... when your phone is on, it won't be downloading data all the time - only when you've got an application running that is using an internet connection. You're charged by the amount of traffic you download, now how long you have a connection to the network for. Just thought I'd try and clear that up in case others got confused from what was said before...
Oh, and yes, Virgin use the TMUK network for all of their services that a customer would see, internal services (such as billing and provisioning) and customer services are provided by Virgin direct...
HTH,
M.
entirely true, however the iphone continuously checks email etc.. therefore with out unlimited plan you will be using the internet alot, as was the problem with the guy who took his iphone abroad where thre is no unlimited data, he didnt use his internet but still got charged as a result of iphone checking
on a side note o2 suck, its too expensive in the uk im not paing £899 to have a phone for 18 months its ridiculous, ipod touch for me with a new cheap contract as well, not that you care but my two pennys nevertheless