UK - What constitutes dangerous driving?
What metrics can I use to make a judgement call about whether a vehicle's actions are actually making me unsafe

The UK Highway code says they should give you as much space as they give a car, illustrations in the code suggest they should move completely into the next lane. At least one UK police force uses 1.5 metres as the minimum separation.
My old copy of the Highway Code has this illustration:

It appears that failing to give enough space to a vehicle being passed is usually prosecuted (if at all) as Driving without due care and attention, not Dangerous driving.
The Criminal Prosecution Service says:

There are decided cases that provide some guidance as to the driving that courts will regard as careless or inconsiderate and the following examples are typical of what we are likely to regard as careless driving:

driving inappropriately close to another vehicle;



The Highway Code
The UK Highway Code says

Rule 163
Overtake only when it is safe and legal to do so. You should
...
give motorcyclists, cyclists and horse riders at least as much room as you would when overtaking a car (see Rules 211 to 215).
Rule 212
When passing motorcyclists and cyclists, give them plenty of room (see Rules 162 to 167). If they look over their shoulder it could mean that they intend to pull out, turn right or change direction. Give them time and space to do so.
Rule 213
Motorcyclists and cyclists may suddenly need to avoid uneven road surfaces and obstacles such as drain covers or oily, wet or icy patches on the road. Give them plenty of room and pay particular attention to any sudden change of direction they may have to make.


The Road Traffic Act 1988
The Introduction to my old copy of the Highway code contains this section:

This code, between pages 3 and 52, is issued with the Authority of Parliament (laid before both Houses of Parliament June 1992)
A failure on the part of a person to observe any provision of the Highway Code shall not of itself render that person liable to criminal proceedings of any kind, but any such failure  may in any proceedings (whether in civil or criminal and including proceedings for an offence under the Traffic Acts, the Public Passenger Vehicles Act 1981 or sections 18 to 23 of the Public Transport Act 1985) be relied upon by any party to the proceedings as tending to establish or negative any liability which is in question in those proceedings
the Road Traffic Act 1988, Section 38

Section 3 of the Road Traffic Act 1988 says

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.


Enforcement
Obviously, enforcement is diffcult and often insifficent. One example provides a guideline for the actual separation that is quantitative rather than the qualitative guide in the Highway Code.
An article &quot;Drivers who get too close to cyclists face prosecution&quot;, The Telegraph 16 September 2016. says:

West Midlands Police force is thought to be the first in the country to proactively target motorists who flout laws requiring them to give cyclists at least the same space as vehicles when overtaking.
Anyone encroaching inside a safe passing distance, widely considered to be a minimum of 1.5 metres, runs the risk of being prosecuted for driving without due care and attention.


Video giving advice to drivers
See How to Overtake Cyclists