If you take a look at the higher altitudes it all becomes a bit clearer - there's a 'spine' of airways running down the country connecting with the 'tracks' that airlines use to cross the Atlantic. The class D sections you're seeing effectively join up with this higher stuff which then joins with all the class A above FL195.
Down in the levels we fly at you are typically only seeing the bottom bits of this stepped system, hence random bits of 2500-3500 class D for no obvious reason.
Apologies if I was a bit harsh but it did come across as a very strange question for anyone that's passed for the PPL - you've got to expect a little Mickey taking.
Bit concerning about the 'no PFLs before the QXC thing' - what was the brief for if the donk failed?