Seems as though all pre-paid data plans are total rip-offs. All searching I've done on the topic seems to amount to: not worth it. Usually they are Pay By The MB. And with quite expensive rates. I dont know how it is in the UK, but from what I've read it's the same everywhere.

I've been interested in connecting a USB 'mifi' to the tablet too, I hope someone who knows about this chimes in.