I imagine any reasonable traffic law would forbid drivers from impeding traffic like that. You might be able to do it somewhat discreetly, but it's still a matter of enforcement, and you can bet blue nonces will suddenly start caring about enforcing traffic laws.
Edit:
UK Road Traffic Act
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.