BB DTY deps. are a pain. 95% of the traffic on TC Midlands(130.92) is going North either off the London TMA or into BB/NX, and the DTY's are going East towards B317/R77, effectively head on to all these.
The outbound agreement is climbing to FL 210, and FL 180 or above by the Eastern edge of B4 (about 10 E of GW).This is to avoid TC Northwest, TC Northeast and TC Capital, who don't know about you and don't want to know 'cos they've got plenty of problems of their own. So we can't just leave you down til later.
Unfortunately, the DTY CTA is only about 45 miles across, so after DTY, the Eastern half has just over 20 miles to put every TMA departure in to, Northbound, 5 miles apart, so it's very difficult to pick a heading to deconflict you from all these guys, hence you climb underneath them, and if necessary, 1000ft a time. We have to keep inching you up, because naturally the next lot of TMA departures are trapped under you, and they too have level restrictions to meet to avoid Manchester's airspace.
Yes, it's crap, but until someone decides commercial aviation is important enough to give us some more controlled airspace (how many PPLs are mooching about above FL 100 anyway?) that's the way it is.
Please please come and have a look at LATCC, it might be an eye opener!