Here, if there was a car coming the other way I wouldn't try to overtake even one single file cyclist.
Once the oncoming road was clear I'd be over in the right lane so they could be three or even four abreast.

Look
A bunch of 3 x4abreast would be much easier and safer to pass than a bunch of 12 in single file.

Norwich Rd
https://goo.gl/maps/f1okBmUAzUK2