Ah, the "I don't much like the cut of your jib" offence. I have problems imaging an offence more open to abuse or difficult to defend yourself again, since it's worded to rely only on proving the copper's belief, not on the actual facts of the riding.
PRA 2002 Section 59
Vehicles used in manner causing alarm, distress or annoyance
(1)Where a constable in uniform has reasonable grounds for believing that a motor vehicle is being used on any occasion in a manner which—
(a)contravenes section 3[*] or 34 of the Road Traffic Act 1988 (c. 52) (careless and inconsiderate driving and prohibition of off-road driving), and
(b)is causing, or is likely to cause, alarm, distress or annoyance to members of the public,
he shall have the powers [to stop, warn or seize] set out in subsection (3).
[*] RTA 1988 Section 3
Careless, and inconsiderate, driving.
If a person drives a mechanically propelled vehicle on a road or other public place without due care and attention, or without reasonable consideration for other persons using the road or place, he is guilty of an offence.
All that said, the chaps blocking the outside lane were doing so on their own cognisance, had been asked politely not to, I have zero sympathy with them. Your ride, your risk.