The law on projecting loads is that they must be clearly marked (with white and red diagonally-striped triangles if they protrude more than two metres behind the rear lights and reflectors.

…which means one shouldn't get within two metres of what one perceives as the back of another vehicle. But that's probably good advice, anyway.

I think it should be less than two metres, until it is also the law that all on-road cycle lanes are at least two metres across and it is illegal to park motor vehicles in them.