i flew from duxford to stapleford a couple of months ago and got transit right over the 23 numbers and their tower at 1500ft! just decided in the air that we would try our luck or just go between the luton/ stansted zones.
if i were you i would give them a ring before leaving to ask how busy they are or even ask cambridge tower to contact them before you takeoff so they know to expect you.
head down to BKY and contact them ASAP. ask for zone transit routing BKY to chelmsford. that will have you clear of approach and departure as your at 90 degrees to the runway. its a great view when you see a line of ryanairs ready for departure and a 747F taking off right under you!!