I don't know the situation in England, but here in Holland you need to pay for a landline to have DSL. But it is possible to only have an internet connection without telephony, wich is cheaper than having both. I only have DSL through KPN, and use VoIP and mobile for phonecalls.
There are definately differences in VoIP providers. One thing is that Skype is not like others, they use their own standard and technology. This means only Skypers can call eachother for free. With a lot of other providers it doesn't really matter who your provider is and VoIP to VoIP calls are free. Also, I don't know if Skype gives the possibility to 'buy' a normal phonenumber. People can call me on a normal phonenumber and pay normal charges for that, in stead of paying up bigtime when they call me on my mobile phone.
I think calls to mobiles are expensive with any provider.
Voipbuster lets you make free calls, but there are (how surprising :p ) some limitations. First, you must buy some prepaid calling money (how do you call this in English??) to get 120 socalled 'freedays'. I don't know what the minimum amount is. Second, the free calls are limited to 300 minutes per week, looking at the last seven days. After that, calls will be paid for with your prepaid account. But even with those limitations they seem to be pretty cheap.