I've already done two this year at Oulton Park, but it's probably better to just pick one day & track from the multitude of track day organisers and go with them. Depending what day of the week you choose and what track, you're looking at £100 (midweek airfield) to £300 (Saturday/Sunday Silverstone).
My preference is BookaTrack and they do plenty of open pitlane days so you do as much or as little as you want.
Just got to sort out a venue and date that the majority can agree on, and then book via the organiser's website (not necessarily Bookatrack of course).
At least doing it through 3rd party you don't have someone on the forum shelling out £15k and then trying to collect £150 from 100 different people, plus you'd have to organise public liability insurance, ambulances/medics, marshals, fire tender, butty van, etc.
Alternatively you could just go to one of the BMWCC events (they've even got one at Croft).