To go back to the op and group rides doubling up being 1 illegal, 2 unsafe and 3 inconsiderate
1 it's not illegal the highway code clearly says do not cycle more than 2 abreast
2 it's not unsafe as it's easier to see a group of cyclists riding 2 abreast than to to see them all line astern.
3 it's not inconsiderate as it makes them easier to pass, simple maths. To get past 1 cyclist requires a pass where the driver is exposed for aprox 3m plus the distance required to make the pass driving past a line of say 12 cyclists that becomes 36m plus. whearas passing a doubled up group that distance is halved to 18m+ thus halving the drivers risk and making it easier to pass as he doesn't have to wait for such a large gap to appear to enable the pass. I would argue that in a group ride that makes it more considerate to ride 2 abreast.
I agree its not illegal.
In some situations it is more safe and to be recommended on that basis.
In some situations it is not inconsiderate.
In some situations it is inconsiderate. Where motorists would safely be able to pass if cyclists singled out but not if cyclists are 2 abreast it is inconsiderate to ride 2 abreast.
Simples.
Furthermore i would regard it as advisable if cycing in large groups were discouraged on public roads. There is not benefit to be gained for anyone from cycing in large groups and there are many associated problems including safety considerations, public relations and public nuisance.
Id say max of 4 to 8 would be acceptable on public roads.