Is it acceptable for a group of circa 15 cyclists to cycle single file without leaving any gaps?

Whatever the configuration of the 15 cyclists, on most roads, if you've got a whole bunch of oncoming, you aren't going anywhere past them in a hurry. No cars/less cars = no problem/ less problem. 

Cycling in a group or cycling alone, there are a minority of drivers who will always consider your presence on the road to be inconsiderate.

https://youtu.be/mzezPD1JpjA
https://youtu.be/nS1Rnwjzdl4

Neither is it just cyclists who are a target for their aggression, evidenced by no end of driver on driver road rage videos.

I feel you have bought into the whole manufactured nonsense of driver v cyclist and the myth that cyclists are a significant cause of traffic delays. You are seeking to address this "problem" by insisting that the person on the bicycle never makes an error of judgement and always prioritises the driver in any shared road use situation. You can dress that up as common sense, cyclist will always come of worse, shouldn't use the road for recreational activities etc, but maybe you should focus on the inconsiderate and far more hazardous actions of the angry driver demographic.

AndyIT wrote:

2 groups would make it easier and safer to pass for all concerned; we have to share the roads, can't see a them and us attitude helping

One group riding 2 abreast would be even easier to pass but most drivists would complain about having to cross the centre line. Riding 2 abreast would also make it easier to communicate up and down the group so the ride leader could make a decision about what best to do quicker.

Just for clarity, this is not particularly for convenience of motorists. If anything 2 small groups with a gap can be more difficult for a low ability driver to cope with than one larger group. It is more that we are not professional cyclists used to bunch riding beyond weekend club rides. Also not riding on closed roads, so groups of 8 is just easier for us to keep organised, keep together and ride neat and tidy.

I've been stuck behind a large chaotic group once or twice, for about five minutes. Not the most annoying, but I would expect any group who knew what they were doing to go side by side or split into smaller groups. The last time, the leader looked embarrassed when they finally organised to let me pass.

I do pay tax like a car, free less than x  g/km of carbon, like my petrol car is free. We're registered with British cycling, and I have third party insurance. So you're saying we should ride in a 30 peloton, great news! 😍😍

Yes, did overtake a club once on a mass ride. They were in separate, large groups.