Interesting! To summarise this thread over the last 24 hours then, we've got one poster seemingly arguing that there's no need ever to let someone out at a junction (since when have you only had to give way to the right at a t-junction by the way?), and hence the fact that you should never ever use a flash of the headlights to indicate that you're letting them go won't be a problem. That poster than goes on to suggest that you'd be at fault if you flashed to let someone pull across in front of you who then collided with someone else, even though - in the same post - a flash of the headlights serves only to make them aware of your presence. So - according to that - you flash your headlights to make them aware that you're there, but it's your fault if they interpret that to mean something else and they pull out into oncoming traffic? Meanwhile, in the real world, I can think of many, many examples of times when I've been grateful that someone's let me turn right across them to fit into a rare gap in the traffic I'm trying to turn into, saving me potentially a few minutes at little or no cost to themselves (or vice-versa). Sometimes a dose of reality can be quite enlightening.
Then the old chestnut of speed limits not being a 'target' comes up again. If it's safe to drive at the speed limit for your vehicle (and the vehicle is capable of achieving the speed limit!), then it's pretty inconsiderate to do otherwise for long distances without pulling clear to let the inevitable queue disperse. I can live with that though - as others have said though, it's those who drive at a steady 40 or 45mph regardless of road conditions or the prevailing speed limit that irritate me. I'm sure some of the examples of dangerous overtaking mentioned earlier are the result of frustration so caused.
Finally for now, going back to the original post - a few years ago now, I followed a 4x4 on a fairly quiet main road (just the two of us, noone else in sight). Bigby High Road out of Brigg, for what it's worth. Somewhere around the left turn for Kettleby, a tractor is waiting to pull out of a farm on the left to go the same direction as us. Despite the fact that there's noone visible for some distance behind us, the 4x4 slows almost to a stand to let the tractor out (we'd both have been past anyway by the time the tractor driver realised what was going on!), and we then have to follow it at a crawl for some distance. Letting someone out isn't always the most appropriate or courteous thing to do!