If you can live without video calling, it might be worth trying out a full blown VOIP service. As you say there are plenty of services out there and after trying a few, I've been using yay.com for a year or so. Its a superb business grade VOIP service with excellent reliability/voice quality and with great support - probably the best phone support I've ever received from a UK company. You are not forced to use a POTS or IP phone with yay, they offer a website/desktop app and more recently a very nifty iOS/Android smartphone app. The app allows you to do things like use your calling plan, record voice calls, download answer machine messages and tons of other features. They're not the cheapest out there but not the most expensive either. I'm on their PAYG service for £2.99 p/m with call charges on top, but they offer a UK unlimited call plan for £9.99 p/m or with mobiles included £15.99 p/m. When I signed up, they offered a free 30 day trial with £20 credit included, though not sure if they offer something similar currently.
Yay (like most VOIP providers) can transfer your existing landline number over to their service, I think i paid £20 for this.
I didn't have a great experience with cheaper VOIP providers such as Sipgate Basic whose service was patchy (too many outages) along with p!ss poor support (Germany based). If you make a lot of overseas calls, then you should also consider Vonage as they seem to be very competitive for overseas call packages and they do have quite a good reputation for VOIP services.
Hope this helps
FluidOne FTTPoD 330/30 Mbps