Hiya,
I've mentioned this before, and like Aless says - doing a Yahoo or Googly should turn up plenty of other providers -
However, I use Swiftcall. I pay a flat rate of £19.99 per month, and ALL my UK landline & Geographic numbers, USA, Canada, Australia, about 15 European countries are all FREE. There's an 0800 freephone access number then a PIN to enter. You need to register the actual landline you'll be using with the service and it's the only one I can find who also allow a Mobile phone to be registered as well.
It's great for 'fixing' costs - the only other charge is Line rental from BT (or your cable provider) and 0845/0870 and mobile numbers.
Cheers! DtM - West London & Slough UK!