After reaching A060 on departure, there are so many conflictions in the way of LHR inbounds at 1000 above, and in/outbounds from Luton and Standstead etc that it takes a while to clear them. On the off chance tho sometimes when there is a break in the Lambourne inbounds, you can get higher climb sooner, but usually it takes a few miles on the northerly heading to clear all the mish-mash of traffic in the London TMAs