Are you sure that price is right?
OK it is new price plans...
http://vonage.co.uk/specialoffers/index_uk.php
If your Cable Modem is plugged directly into your PC then you will need more than their free hardware (one of their routers with built in VOIP as MattD said. The Linksys one is very good. I have used that before). If you have a Cable Modem compatible router you plug the widgit (made by DLink IIR) from them into it and away you go. This will mean you need to move your cordless phone base station to near the PC.
It does not work with Sky/Sky+ so if you have that as well be aware.
Call clarity is great. My UK box now lives with me in the US so anyone in the UK can call me on it using a UK number. I have set VOIP traffic to have priority on my router and have a 15/2 connection, but even while downloading from newsgroups call clarity is unchanged. The power adapter for the DLink is 100-250v so can be used globally with a plug adapter on the power socket. The other end is just a RJ45 Ethernet plug so works in any router.
So how much is the phone line element of your VM bill? More than 5.99/7.99 a month?
A final thing to note is that at the moment there is no number portablility to Vonage. You will get a new phone number. Probably in your area code, but it may bear little relation to phone numbers that you usually see in the town you live in. For example most phone numbers in the 01992 area are either 4xxxxx 5xxxxxx 8xxxxxx. Vonage numbers in this area are 6xxxxxx