Two out of the five major carriers in the UK offer genuine, 100% unlimited data (including Tethering):
Three and T-Mobile.
T-Mobile merged with another network (Orange) in 2010, and combined they are now the largest. That suggests to me that offering unlimited has little to do with them needing new customers.
Three is the smallest network (but growing at the fastest rate), but they take the attitude that capacity problems cannot be solved with monthly data caps*
Giffgaff as mentioned earlier is an MVNO that offers "unlimited" data, but Tethering isn't allowed and it's said that customers will be disconnected if they regularly use "too much" data. They only offer prepaid plans as well.
New advertising rules brought in at the start of April mean that you can't offer a product that is "unlimited" unless it genuinely is. Any hidden caps, fair usage policies or throttling must be prominently advertised.
*If you have 100 people all wanting to use 50MB of data in the same place at the same time, no cap is going to help with that. Three uses demand based traffic management. Each cell site is individually managed, with customers only being throttled when the site they are using doesn't have enough capacity.
I should note that both T-Mobile and Three pitch it as part of their "everything" packages (T-Mobile's "The Full Monty" and Three's "The One Plan", where you get everything you could "possibly" need included (although that's really not true). It's not something everyone gets as standard.