Even though you can use Vonage in the UK now with a broadband connection, officially they are not there. Well that will change as they have announced their intentions to be there come this December it seems.
First they will only be selling via their web site, but then they will migrate to retail, following the patter they have established in the USA and Canada.
I predict that they will not be the only VoIP player from the USA to enter the British market.