Sometimes I think drivers like you to make the decision for them, if left with a dicey overtake when a cyclist is in secondary they feel duty bound to take it, if you are in Primary they will tend to accept it and realise they will have to wait, or least most will, Alpha males in Silver German cars notwithstanding.
That said, I sometimes drive and I would get mightily peed off sat for eight minutes behind a cyclist doing 15 mph, I wouldnt do anything rash but I would be sat there, annoyed and wonder why the cyclist is hogging the entire lane, if it were necessary due to parked cars etc then not an issue but I have seen unrepentant Primary cyclists and they are annoying, like getting stuck behind a dawdler in a car, one of those who is either unaware of the queue behind him or is making a point.
So, ride Primary when your safety is based on doing so, otherwise ride secondary (not right in the gutter), make plenty of opportunities for drivers to get past and make the odd gesture of thanks, I find it can even placate a German car driver as you are acknowledging their patience and improving their view of cyclists so the next one they come across may get treated better.
I find a bar end mirrror is invaluable, I wouldnt ride without one now, saves looking over the should ehen you want to know whats hoving into view behind you without the turning round, its much quicker, as long, of course that you remember to look around for making a maneuver or for reminding those behind that its a bloke on the bike, not just a hi viz jacket.
I think sometimes looking a bit "official" helps as they arent sure if you are a PCSO or something, or at least looking like you arent totally out of your depth.