Well, since this thread was created, Virgin Mobile now offer 4G - though speed capped to 25Mbps (or so it was when I last checked). No FUP either as far as I am aware, so you won't get capped to 384kbit/s anymore.
I am also no longer with Virgin Mobile - due to James_W's actions, among other reasons, I am now with EE and have been with them for over 18 months. I get full 4G, including 4G+, as standard and have 12GB of 4G data for just £14 per month. That's less than what Virgin's 3G "unlimited" (3.5GB) SIM even started off at!
if I had a compatible device/firmware, I would also get Wi-Fi Calling, 4G Calling, and access to EE's low band 4G, which travels further than standard 4G (and 2G/3G). Virgin Mobile customers certainly wouldn`t get any of those features now, or for the foreseeable future.
My rating for Virgin Mobile, is of course, bad (and not recommended). Any other mobile network is a better solution.