Muarrij. I recommend by all means using an SIP compliant phone. I am just in the process of shopping around now for the best deal. I guess Europe has not gotten awake on the VoIP issue. So I believe you will not find a competitive price in the U.K.
Where do you want to call with VoIP. If you want to dial a POTS telephone, you will need a gateway (Internet telephony provider) such as iconnecthere.com (my personal best “look and feel”, never actually used) or vonage.com or packet8.net. If your aim is to call a friend who has a similar SIP device, you are invited for free to use fwdnet.net (pulver free as mentioned above in the thread). All what this service is doing is keeping track of your IP address and linking that with a personal “IP-phonenumber” given to you. If your friend is using the same service you can dial his number instead of his IP address and you don’t need to hassle in order to find out his ever changing dynamic IP address.