In terms of grid numbers your best bet at Llandow is the Prokarts, the grid numbers are growing constantly and boys like Nigel and Tutty are responsible for most of the newcomers. They both are friendly and give good advice and help. You'll find yourself surrounded by lots of friends who turn compettitive once on the track and back to friends once off again.
In terms of race enjoyment and sheer power/speed/acceleration you'll have to try the Rotax, Biland and the TKM 4stroke. Lighter, faster and better acceleration. However, at Llandow all those classes seem to be in decline at the moment, one reason being the higher costs, more frequent rebuilds and bigger difference in performance between those with money and those racing on the budget.
As Nigel said, come to a race meeting, watch some racing and have a chat with people in all the boats, and I mean all of them. See if you can blag a drive, you never know (it won't be at the race weekend) you might get lucky!
Occasionally kart re-sellers organise test days with reduced purchase fees. It is a good way of getting to know and drive different chassis/classes/etc. Biland had such a test at Llandow recently and they're now moving on to Rye House in Heartfordshire next on the 12th June. I don't know the inns and outs of the deal but there's no harm in asking, if you want details drop me an e-mail and I'll forward the info to you.
Another good place to keep an eye on is the UK Karting website, very good noticeboard, second hand marketplace etc.
www.karting.co.uk
Dan