I moved to virgin mobile a month or two ago but recently I've found that 3g (or any data access for that matter) has been virtually non-existent in central London.
I use my iPhone for syncing my emails, a bit of mobile browsing and checking bus/train/tube times/delays. I don't stream video and I don't download large files. In a typical month I'll use perhaps 500mb of data.
My experience has been that virgin is useless for data. 90% of the time I find that while my signal strength is fine (and I can make/receive calls and text messages) I can't connect to web pages or use apps that require a data connection. Typically the connection times out or I encounter proxy error pages (clearly Virgin run transparent proxies to cache data within their network).
While I was with O2 (with the same data usage) I was always able to connect - it might have been slow at times but it always worked. Given I'm on a monthly contract I'm going to be voting with my feet and moving to a new network next week. Just need to decide who!