IPhone :: 3gs Update With Unlocking?

I have an iPhone 16GB 3Gs that I bought off a USA-eBayer a while ago (though I live in the UK). I'm with Virgin Mobile that 'does not' support the iphone, though when I rung up before the purchase of the phone, the team said though they don't stricktly support the Apple device, but iPhones generally work on their network.

I bought in confidence and my iphone worked floorlessly for about 12 months on the iOS 4.1 system, becuase I didnt want the hastle of unlocking my phone back for the Virgin Mobile website each time I updated through iTunes, i didn't bother. Evenetually I updated to iOS 5.0 (off the internet rather than iTunes) and since then my phone has really struggled to keep a strong signal, or indeed WiFi connection. It can have full signal but then it says 'Searching...' for a couple of seconds then will find all the signal again. This as you can imagine gets pretty infuriating when its happening 4 times a minute, and only when I want to actually use the phone.

Also the phone likes to turn off at any point, even if it says it has about 70% battery and was on charge all night, then it demands to be on charge before it turns back on again, sometimes stating that it has the '70% charge' it told you before, or a dramatically reduced charge depending on its mood.If I wasn't worried about it getting locked to the original carrier, I'd just plug it up to my iTunes and update it, but as it stands, I cant do that!

Info:
iPhone 3GS, iOS 5.0.1, 16gb white