I think the problem is that both cyclists and horse riders are advised that they should ride two a breast so that it forces traffic to slow down and wait for a clear bit of road, instead of squeezing past a single cyclist or rider. I have often wondered at the selfishness of this advice, and the fact that it will and has, caused tremendous resentment towards them.
The advice is not selfish, but some peoples' interpretation of it is. If you follow the highway code, you need the width of your car, the same space between you and the cyclist, and they don't have to ride in the hedge either. So if the road is narrower than that, it's perfectly reasonable for a cyclist to ride in such a way that discourages overtaking.
I'll be the first to admit, it's a lot of space to leave a cycle, and I'm happy with less, but not everyone is. Having said that, my three wheeled pedal cycles usually get left far more space as they do look a bit odd if you're not used to them.