i would say use your own car TBH, but be carefull which type of trackday you choose to do your "first" one on, keep clear of the speed fest or speed sundays they do at knockhill as the driving can be quite eratic to say the least, ive done them ones before and never had any bother but thats probably cause i had one of the quickest cars there
i would recommend the SIDC trackdays or the trackscotland ones before any other, think trackscotland have one going tomorrow TBH