You get put on a heading when flying on a DVR out of Luton so that we can get the London City inbound through your level and down to altitudes and/or to parallel you off with the Stansted DVR departure which always seems to get airborne at the wrong time. You then get climbed to "Min Stack" which is FL70 or 80 depending on the pressure.
As I'm sure you are aware, the Luton DVR SID takes you underneath the LAM holding area for Heathrow which is the busiest for Heathrow which is why you don't get climbed above FL70 (or 80) until I suggest DET not DVR.