single file and cars can go past without oncoming cars having to stop & let them through. Common decency IMO.
I'm not on about what is legal....I am thinking about what is best for other people.
@Dave7 , if they singled out, it would be harder for cars behind to get past as they'd, driving prudently, need more clear road. What regularly happens, with a singled out group, to cries of "Car Up!", is that the overtaking driver gets part way, sees an oncoming vehicle, and then 'cuts in' with explicit increased danger to the cyclists 'in the way'. So actually the cycling group IS possibly thinking what's best for the cars behind (a point previously made). I think for the reasons others have offered (overtaking car doesn't have to go in the other gutter), two abreast would be a bit more sensible/sensitive and achieved the objective (primary position qv), but again, the OP was behind and it may have just looked as if they were 3 abreast. Whatever, any car passing them safely would need to go well or even completely over the centre line (marked or otherwise) of the road.
It isn't a piss take. It is a cyclists way of saying "I'll determine how to manage the danger you represent to me thanks, because you will always make the wrong call. And my risk is HUGE."
As a lone rider, this danger/threat management is best achieved, in circumstances like these, by assuming the primary position whenever there is increased risk to the rider, of a car coming past without sufficient visibility to see 'round the corner'. On a single carriageway road approaching a bend I often but not always adopt the primary position (tends to depend how narrow the road is). This ensures that any driver behind desiring to overtake me, needs to make a conscious decision to pull into the lane designed for traffic coming the other way, risking - for the driver - a head on collision if he/she gets it wrong, as opposed to the cyclist in the gutter and squeezed when the overtaking driver suddenly sees a vehicle coming the other way. As soon as I can see round the bend and it's clear I'll pull back to the normal distance from the gutter/kerb.
As an experienced club rider we only ever went two abreast and singled it out for cars that were struggling to get past.
@Dave7 I'll make a guess that your cycling in a group (ie on a highway, often two abreast) has been limited. If not, on your club rides (or whatever), did everyone cycle along in single file?
What about your group riding experience
@coco69 ? Has your club ride not held up traffic behind a bit? If the group was only two abreast, would you not have been so aggrevated?