AFAIK 3 are the only provider in the UK to offer unlimited mobile data.
Do neither Orange or T-Mobile offer unlimited caps? To be honest I've never hit my cap on 3G so don't see it being a problem with 4G.
4G will be a complete waste of time. The only uncapped provider doesn't have it, so it'll be a case of basking in the glory of buffer-free HD mobile video, for about 30 seconds at the start of every month until you hit the cap.
I don't really care who I'm with tbh. And 4G will be great to have.