Not only enforcement, and let's be frank, enforcement of speeding, parking, any other antisocial behaviour on the roads and antipathy towards more vulnerable road users is on a sliding scale of hardly a deterrent down to just not done at all.
To play the defence lawyer for a moment:
But say you do get an official interest, 1) How do you prove beyond reasonable doubt that the vehicle was too close without verified measurement of the closest point, it is A's word against B's.
2) How does the cyclist prove that their line was consistently, unwaveringly straight and they didn't accidentally wobble or veer towards the vehicle, making the pass illegal even though the driver didn't change course and was legal when they started the manouvre, even the HC warns drivers the roads are not perfect and bikes etc may make unexpected course changes to accommodate this problem. Bikes are also inherently less stable and more prone to a good gust of wind or the turbulent air a bigger faster vehicle creates.
3) Yes bikes are advsed (not legally obliged) to primary and secondary position etc and HC say ride at least 1m from the kerb (measurement again) but my client is adamant that the cyclist was deliberately riding too far into the carriageway and seeking to cause an obstruction or impede other traffic by their actions.
As a deterrent it'll be not even ignored, it'll simply not impinge on peoples consciousness, police will give it less credence than ASLs, and as a provable prosecution with a legitimate chance of conviction & VFM & in the public interest, the CPS won't even take the time to laugh at it.
It is a nice and laudable and morally correct idea but untenable in practice in the realities we have right now.
A law that would prove a far more effective deterrent to close passes would be one absolving cyclists from any legal liability or financial penalty for damage caused to motor vehicle if they fixed a diamond tipped glass scribe on a 4 foot long pole sticking out of the right hand side of their bike.