Looks like an intermediate altitude assignment from atc. A look at the standard instrument departure plates (SID's) for Stansted shows that the final altitude on some of the procedures is 6000 ft. They probably had to stay at that altitude until the departure controller passed them off to the en route controller who assigned them their cruise altitude.
On the initial IFR clearance, if I'm not mistaken, clearance delivery will give them a time to expect this assignment. The clearance will say something like "climb via [name of SID], expect flight level 350 one zero minutes after takeoff." My wording may be off. The departure controller will assign whatever altitude they need them to be at. Usually what's on the procedure. So they stay at that altitude until the en route controller assigns their cruise alt.
I picked a random SID plate for Stansted. At the bottom of the plate it tells what altitude they have to be at by a certain point. Looks like they have to get to 5000 ft pretty quickly.