There is no point getting worried by the etiquette, as you then just get narked by all the hundreds of people who have never worried about what the etiquette is. My personal London aggravation is people who sit waaay in front of the stop line at junctions, can't see the lights and set off late so I pass them, stop at the next line, they drift in front again and so on. But hey.
A bloke went into my back wheel under Admiralty Arch t'other day when I slammed on the anchors to (I thought) avoid being clotrheslined by a van's mirror. Sorry if that were you.