If you tend to have a landline just for free phone numbers like 0800 you might save quite a bit by using giffgaff which doesn't change for free phone numbers. http://giffgaff.com/index/pricing
There is also ovivo on the Vodafone network which gives you free minutes text and data every month for nothing, you just buy a 15 quid SIM card I can't see how they can keep going though but they have been around for over a year now I think. Just don't port your number over. I don't know if they have free free phone calls though.