Have a look at this thread by Housey about Phil Price Rally School. It's only £99 over your initial budget and appears to be utterly awesome and is the sort of driving day I'd want to do if I were after an experience day.
I thoroughly recommend this to anyone who wants a good full day with lots of seat time and the freedom to have a proper go. I've done it 3 or 4 times now and always book with a group of 10 mates, so have the place to ourselves. I think we pay circa £200 a head to do that and usually stay in a hotel about 20 minutes away the night before for a spot of dinner.
You then head up into the hills in the morning and into the school and spend the day blasting MK2 Escorts around followed by some Scooby fun and then end the day with a competition and a fast stage with Phil in a WRC car, which is fantastic and never ceases to amaze me how smooth a rally car is on the stage.