As you're probably aware, in the UK, 'Freephone' calls from mobiles are charged at a premium rate on most networks. The regulator, OFCOM is trying to change this, but the changes aren't expected to be implemented before 2015.
In the meantime here's a way around it.. PLEASE NOTE this method does not make the call 'free', but you'll be charged at normal landline rate, or it'll come out of any minutes you have included in your plan.

Ok? Save this number to your contacts: 0200 222 0700 give it a name like 'Freephone' and next time you need to dial a freephone number from your mobile, hit that contact and call; a computer voice will say 'please enter the number you wish to call', enter the freephone number using your keypad, and that's it - you'll be connected at your normal per-minute landline rate.

Please also note that it's garion posting this, so it's not some sort of scam, it's a genuine money-saving tip - I reckon it's probably saved me around 30 quid over the last year.