I think Red Herring summed it up. If you're likely to be overtaken, and you're aware of someone wanting to overtake, you should be cooperating with them, without endangering yourself, obviously.
The ones who always amaze me, however, are cyclists. At 5 mph, they insist on riding 2, 3, or more abreast, with no apparent awareness or consideration for traffic held up behind them. Yesterday I passed a horse rider coming towards me. Behind her was a "peloton" of cyclists out on a road race, taking up the entire lane. Despite the horse rider frantically waving to them to stay back until the cars on our side were clear, they pressed forward, in a big group. I couldn't keep looking to see what the horse's reaction was, but a spooked horse lashing out in the middle of a group of cyclists would have a pretty devastating effect! On the way home, we had to contend with 3 lads slowly weaving about ahead of us on bikes, one of whom decided to undertake the other two by popping his bike up onto a grass island in the road and back down again. Again, we had to wait until there was a straight clear stretch and fully offside to pass them. How hard is it to ride in a disciplined manner along the edge of the carriageway?!
Didn't see you Cie, did you have an enjoyable ride? Glad to see North Wilts being ever more strongly represented here