I understand that, but having never really driven anything more potent than this Golf in anger, I just don't feel i'd make the most of it compared to doing it in a couple of years when i've had more opportunity to drive stuff with power, rwd etc. and have pushed them a bit.
This won't be an issue. Bedford has miles of run off everywhere so you won't have any fear of stepping over the limits. The cars are easy to drive and you have an instructor to guide you.
I first did one of these just a few weeks after I turned 17 (just 5 days after i passed my test). The quickest car I had driven at the time was a Datsun Stanza. It wasn't an issue getting the most from the day (in fact I was even quickest in some of the events!)