Personally I've never seen cyclists riding more than two abreast - three or four would be dangerous and breaks the Highway Code.
Two abreast is legal and, generally speaking, safer for riders and drivers taking only marginally more road space than a correctly positioned single rider who should be positioned at least one metre from the kerb.
Larger groups riding two abreast reduces the difficulty of overtaking for drivers as it reduces the size of the group. For example ten cyclists in single file are 50% longer than an HGV with the same difficulties of overtaking. Reducing the length of the group by half makes it simpler for drivers to pass safely.
The Highway Code suggests:
Motorcyclists and cyclists may suddenly need to avoid uneven road surfaces and obstacles such as drain covers or oily, wet or icy patches on the road. Give them plenty of room and pay particular attention to any sudden change of direction they may have to make.
give motorcyclists, cyclists and horse riders at least as much room as you would when overtaking a car