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.