Trackdays are all well and good but too expensive in the UK, over regulated and no less dangerous than a sprint (more dangerous in my opinion). There is no competitive element, timing is strictly banned and if someone falls off the track they get sent home, the sessions get screwed up and also there are usually a few dickheads that do their best to ruin the day for everyone else.
However, Croix en Ternois is not like this at all! and also very cheap.
I think Peter's suggestion of organising a limited number of events - sprints and hillclimbs - is the best idea to ensure a decent turnout.
At least there is a competitive pupose to it, you also have the option of just trying to beat your self . Brighton is a good one as there are no bends! but they can be a bit prissy about giving out entries.
Lydden also a great sprint track, but a long trek down for some.
Peter would be the best guide to courses further North.
Sprints usually safer than hillclimbs as they are on circuits and there is usually a bit of run off.
Racing is fine but is expensive. There are few race series in which the GTA would be competitive and it would cost a fortune to prepare a car.
Sprints are just good social events, where we can have a bit of banter, unhealthy food and drive the cars a bit.
How about trying to do say Lydden or Goodwood (sprint), Brighton(standing 1/4 mile), Croix (trackday), Santa pod (run what you brung type thing), plus a couple up North.