Hi OceanCrosser,
I will try and keep my answer brief. I do work on the sector that you travel across. There are many reasons why and any one of them, then there is a requirement for aircraft to be on a heading as mentioned. You do mention that with RNAV, everything was supposed to be easier but aircraft may be equipped but the route structure is still the same and the procedures that go with it haven't been updated but work is still ongoing. This of course is coupled with the individual in situ and how happy they are for you to be closer to other aircraft either at the same level (hopefully not converging) or climbing you through other aircraft.
Ultimatly, I think the UK pushes 25%ish (latest figures) of all of the EUs traffic and I for one would love the military to just do their thing close to iceland but alas.......with so many MTAs and Danger Areas, we have even less airspace through our upper airspace route structure so unfortunatly by definition you are closer to all the other traffic and with so many airfields in the UK, all the overflights will need to be accommodated with all the inbounds and outbounds into the UK as well as Paris, Amsterdam, Dublin and Brussels FIR. It does get interesting.
The last reason is pretty much for sequencing, so not really traffic but getting you in some order for our colleagues in the next sector.
I think to truely appreciate it, I suggest a visit and you will be able to see for yourself what our issues are and what we are trying to achieve to but a lot of the times, a direct would help but if you could convince the military to go away.....that would be very much appreciated.