I think this might benefit by being moved to the 'general discussions' section, to give it greater visibility to the whole CC community.
But specifying a distance ie in law, however difficult to enforce in practice, rather than leaving the courts if not the driver to interpret what "plenty of room" MEANS, is probably better. Let's face it we/I would be content with a metre: that's noit a close pass and on a narrow road I'm content with closer, on my terms (ie primary position, look for a wider bit and indicate to the car behind that you are expecting, indeed keen that they pass. More in this context is not reasonable in practice and it (ie legal specification) should be achievable. Could be extended to passing distance for pedestrians, btw, including cyclists passing pedestrians.
"Petition To introduce a permanent, minimum passing distance when overtaking cyclists.
"Cycling in the UK has become a truly popular sport and way of commuting, but we still vulnerable.In 2014 21,287 cyclist were injured in reported road accidents in the UK, 113 were killed and 3,401 were seriously injured.The Highway Code, Rule 212 states giving "plenty of room" when passing cyclists.
"The lack of a clear specification may result in a personal decision what a "plenty of room" means in terms of distance. Therefore, introducing a minimum legal passing distance when overtaking cyclists will considerably reduce the number of cyclist casualties, aiding in a safe cycling practice. Suggestion of 3.28 ft (1 m) when overtaking cyclists on roads with speed limits up to and including 30mph. On roads with higher speed limits, the minimum passing distance should be 4.9 ft (1.5 m)."