Curborough might be a little small to have much fun - and there's not a lot of space if things go wrong.
Silverstone is very expensive, but the International or Historic GP circuit are the ones to go for.
Donington International circuit (not the GP circuit) would be a good compromise as it's got some fast bits without two extra heaving braking points that add nothing to the experience.
Bedford Autodrome is a good, safe venue due to having loads of run-off, but it's a bit featureless.
Oulton Park (GP) is my local track and I love it, as I do Anglesey (Coastal) - but they're probably too far north and west respectively for a lot of people.
Anglesey (just after it underwent a huge redevelopment)
And where else would you get a view as good as this?
Oulton Park from my usual spot - that's where Tom Onslowe-Cole decided to ram our marshal's post
The cheapest prices will be mid-week (i.e. Tue/Wed/Thur) which might make it a pain for people who've still got jobs

, and Mon/Fri will be the next cheapest with Sat/Sun being peak prices - although off-season booking are always cheaper than mid-summer bookings.
Depending on the responses you get here (i.e. less than 20), it may be cheaper & easier to get one of the track day companies (Bookatrack are my favourite) to do a 20 minute Z4-Forum only session every hour. That way you don't have to worry about booking anything, and each owner pays directly to the track day company.