A general test day at most UK circuits wihtout instruction is on average £125 BUT you require a race licience to participate - With instruction is about £350 for a day but that is more like 4 or 5 20 minute sessions as oppossed to a couple
For those without race liciences there are two routes
1) Trackdays which again are about £125 with £20 per hour tuition
2) Obtain a race licience (you don't have to use it for racing about £125 for an ARDS course then you can paritcipate in any test anywhere
Of course all these option use your own car - If you want to drive someone elses you'll look at £500 p.d + for a days testing / trackday
You pays your money you takes your choice
Last alertanitive - find a friendly MINI2 member with a race prepped car and make an offer!?!?