Ok, if memory serves (I haven't flown there since Christmas)...
Circuit height is 1000 QFE (1500 QNH). There is a north/south (S?)VFR transit lane, which you can join from the north at the Pirton VRP, cross the centre of the runway to the Hyde VRP, then on to the Hemel VRP. As you come out of the south lane the LTMA is 2500+. The last time I flew from Luton I was in the south lane at 2400 QFE.
Coming from Cranfield do not be tempted to just follow the M1. You wont make yourself too popular with the traffic coming in on 08 or out of 26.
The Luton controllers are a nice bunch. Remember there is a flying school there so they are used to peoples RT not being perfect.