Without examining it too closely, they are following the rules of the road, as the highway code indicates that they should be riding two abreast on "narrow lanes".
- Rule 213: On narrow sections of road, on quiet roads or streets, at road junctions and in slower-moving traffic, cyclists may sometimes ride in the centre of the lane, rather than towards the side of the road. It can be safer for groups of cyclists to ride two abreast in these situations. Allow them to do so for their own safety, to ensure they can see and be seen. Cyclists are also advised to ride at least a door’s width or 1.0m from parked cars for their own safety.
I'm not an active cyclist, so have no particular axe to grind. Happy to be corrected, if I'm misreading.