I was actually going to suggest Voipfone as I'm a customer myself, but the cost of diverting to mobile was the exact reason I didn't suggest it. I find it's ideal for what I need, but it does start to get a little pricey when you call mobiles. Perhaps factor that into the price of estimates though?
In the long-run paying a few extra quid to have a landline number that people can get hold of you on anywhere is not a bad deal, especially if you convert these people to customers on that call. Just ask them to call you on your mobile in future and keep the number solely for new business.
I looked into other landline providers but didn't want to be tied into a contract, have to faff about with them installing a line (and charging me for it) and having to pay a line rental on top.
Voipfone is essentially pay as you go, you just buy the number (£2, and you can select a number in any area code), top-up your credit (you can auto renew) and pay for the calls you make.