OI! Pish orf, I is composing, after a busy evening gardening.

Much as I laud the idea of a GolfGTI track day, I think the biggest practical issue that you're going to come up against is the commercial outfits like BHP, Mazda on Track etc etc. These boys have excellent relationships with most of the tracks around the UK and you're going to go into something in what is effectively quite a crowded market. Even trying to make sure you break even will be a challenge. There's a well established group of nuts on the Isle of Wight, who've done a number of track days around the UK, which have been very successful, but they've just folded down their systems and told everyone to join Mazda on Track - cheaper and very well organised.
I know how hard it is, as I'm part of a heap of people who go to track days organised by one guy. I've been doing them for over two years. He is wealthy-enough that he can put down the cash for the day once he's canvassed for interest and then hope enough people from his list do pay and show up to drive. So far it's worked, but there are quite a number on the list who actively recruit people to keep the numbers up.
Starting another group like that without a wealthy sponsor who will front the payments to the track owners would be a huge challenge, especially past the first event or two, when initial enthusiasm has died back a bit.
Also it's amazing how many people will come over all hot and horny about the idea and then vanish like April snow when the cash is requested (which needs to be in advance) - it is my opinion that this would be a MAJOR issue on this forum, as it's not packed full of wealthy, professional enthusiasts, the majority of members are young, very enthusiastic, but on a painfully tight budget. Luxuries like a £150-200 track day are the first to get bumped.
Haynes, I've done a couple of times and although it might suit folks on here, it's tiny and you won't get much track time. It is VERY funny though, especially for onlookers. You're also not suppose to time on it, either. As an initial punt it might well be a goer, especially if you could organise something (albeit on a smaller scale) like the first couple of years of VanWest, which was held there. There is a field behind the track, where people could camp, but if you're going to be playing on the track the day after you would need to be EXTREMELY sensible about alcohol.
Here's a vid from my last session at Haynes on
EwetubeMost tracks include the necessary insurance in the price, so as the organiser you're only paying one price.
If you need 60 people in a group DRIVING to make a full-day event, you'll need probably AT LEAST three or four times that many who sign up initially.
Just my tuppence worth.
Nick, if you want help/support/etc in this don't hesitate to tap me up.
</voice of reason>