What you want is the minimum number of cones you can get away with, otherwise you'll spend all day picking them up, and it'll look confusing from inside the car. That's what track day organisers try for.
Is it just an open area that is going to be coned out? If so, you'll need to make sure there is enough run-off area particularly on the outside of corners and after braking zones, so if you lose it you won't crash through a line of cones and straight into the path of a car going round the other side of the circuit. Trig's looks OK from that point of view, apart from maybe the right hand end where the corners are a bit close.
Is there someone from Bovingdon that could suggest something for us - they should know the best way of doing it?