Group II Booster seat - definition?

We're coming to the UK with our kids, and have been told we now need booster seats.
The car hire company supplies baby seats, but not booster seats. What do I do at Stansted at midnight?
In China we don't have them - so what's the definition of a group II booster seat. Yes, I know the size of the child required to sit in it. But I need a definiton of the seat?
Can I make some out of polystyrene?
I can't find any specifications on the net - just that you MUST have one.
Thanks.

The following website will give you all the info you need regarding the law and what seats you can get. Change the rental company if they are not going to help you with the correct seat
http://www.childcarseats.org.uk/law/index.htm