I can't answer your question for the UK.
But in the US there's not charge of any kind for the 3G browsing, shopping etc. on the Kindle and other e-ink devices. It's basically included in the cost of the device.
That said, I seldom use it on my Kindle as the processor and e-ink are just too slow and the screen ratio isn't good for the net.
It's doable for text sites like Wikipedia etc., but I don't use those and just never saw a point. But I suppose it could be handy for looking up something if you're out in public with your Kindle and don't have a netbook or smartphone etc. Moot for me as my Kindle rarely leaves the house.