I am not convinced that doing that will get you a "narrow route briefing" that uses those airfield IDs as waypoints.
Your probably right actually. I hadnt considered that. I guess what it is showing me is a narrow route for the bits it does like, and then all the additional aerodrome specific bits seperately. You live and learn... :-)
The other day I flew up to Walney Island (near Lake District), and because in that instance I had listed IFR waypoints and VOR's on my VFR flight plan, and because I was announcing and flying flight levels, the Doncaster contollers allowed me to transit their class D airspace. This was easier than ducking below the CAS and popping back up the other side en route. Subtle evidence that as you say, you might get a little bit more help from ATC if they think you wont interfere with other aircraft under their control.