Anyone who flashes if you ovetake is a tw*t. Plain and simple. They do it to everyone who dares to overtake them while they dawdle along at 40mph on a 60mph A or B-road. The reason why is because they honestly believe that overtaking is wrong. The highway code says that all drivers must regularly check their mirrors and make sure they are not slowing down the normal flow of traffic and if they are causing a tailback they should pull over to let people pass.
Its the same with chevrons. My understanding is that chevrons are not in any way prohibited from being driven on. The most common use for chevrons seems to be where you have an old A-road that was built nice and wide with at least 3 lanes width. So of course people use it that extra width to safely overtake.
That cannot be allowed by local councils because the overall road planning strategy nowadays is to slow private motorists down so much that the drip-drip effect (of double yellow lines, congested towns, inability to overtake, 50mph limits on open straight roads in the middle of nowhere, speed cameras, road calming) gradually persuades everyone to use public transport. Chevrons are another weapon in the hidden public transport agenda to make people doubt their ability or the legitimacy of overtaking the arsehole crawling along in front.
In fact all you need to look out for, which is what we all learnt on our driving test, is solid white lines. The rule is simple - solid white line: do not cross. If you look at chevrons, most still have dashed white lines because even though they would like to trick you into not daring to overtake on a lovely wide clear road, legally, you are still entitled to, and therefore they have to retain the dashed white lines on each side of the chevron area. I see chevrons as a lane separator which acts as a warning that there may be traffic coming the opposite direction.