I think the STAR says 'EXPECT' 80 HON, which is for your planning purposes and NOT a clearance. Yes on 15 that's lower than you'd probably like, but then again on 33 it's higher presuming you'd like a straight in, so swings and roundabouts. I'm afraid it comes back again to the fact that there are other aircraft in the sky, and that the whole UK system is built around traffic being in places and at levels that offer a compromise and keep most people moving most of the time. If you float down from 220 HEMEL, as some do (especially DLH/KLM to name a few), and are still around FL190 passing abeam WELIN, then it makes it rather awkward for the COWLY sector to work independently (dropping TMA inbounds to 150) of your presence as you turn across for HON. This is why you sometimes get the rather terse 'expedite/increase rate/ minimum rate of...' descents.