I'm with 3UK, and have rang them up to get this filter removed. The person on the phone assured me it would be lifted within 24 hours - it's still the same. This was about 5 months ago.
When I tried to do it, they added a £5/month thing to my account, but then also added a £5/month goodwill, so I wasn't paying for it. I have no idea what it is doing, but there is no filter anymore. I had to phone a few times before it worked though.
Be careful with that, I had them do the same thing to me (after I asked them not to) and the goodwill suddenly ended after a few months. Took hours to get it sorted and refunded, and to top it off they put the flaming restriction back on!
Edit: I just checked, and it looks like it was removed again at some point, possibly when I renewed the contract? Ah well.