Where are you normally based? Wherever that is, I would look at VoIP providers local to you that provide a soft phone client.
A UK-based VoIP provider will expect a UK address for billing and emergency call location (this may not be insurmountable, but was something I came across last year). Of course, once up and running, you can access the service when outside the country, but for setup you probably need to go with a local provider.
SD