Used to do quite a lot of track days in the Carlton. Have done Castle Coombe, Silverstone, Donnington, Brands Hatch, Croft, Snetterton, Goodwood, Marham and a few others.
Most of the better organised ones will spilt you into 3 groups based on your declared ability, although they will promote/demote you if you're obviously out of your depth. Each group typically gets 15-20 minutes of track time once per hour. The afternoon may turn into open pit lane, which means you can go out as often as you like for as long as you like. This is normally because in the afternoon there are fewer people - either because the others have crashed, broken something, run out of brakes/tyres, or just got bored.
They are NOT races - you will get thrown out if you break the rules. These rules include not overtaking near or in corners. You are basically there to enjoy your own car without putting anyone else at risk. There are often instructors available, but really I only found them useful if you couldn't quickly work out the correct racing line. Donnington was the most tricky circuit to learn in my opinion.
Remember that if you have to drive home afterwards, you cannot afford to break or wear anything out on the track. It gets expensive very quickly - I was getting through a set of tyres (£600), a set of brake disks and pads (£1000) and god knows how much fuel (£200?) per track day. Plus the cost of entering (typically £150-£300) and getting there and back (Croft is a long way from Somerset). And those were the days when noting broke.
Oh - and your road insurance and RAC/AA cover probably won't be valid on the circuit. If you break down or have an accident you're on your own.