Challenge it, get plenty of evidence.
Chances are they won't even show up to argue the matter anyway.
I'm not sure about the cops where you are, but up here there's been quite a lot of cases where cops have taken people for non-standard noise-level exhausts (some quite timid), stretched tyres etc. and will go to court, and rarely seem to lose (solicitor dependant).
I had a lengthy chat with a long-time served traffic cop yesterday; the only reason people say 'grey area' regarding the law is because it covers a spread of potential violations.
His advice to stay completely above the law regarding wheels/tyres was as follows:
1) Keep within the vehicle manufacturers supplied wheel size (so from their basic range to sporty range).
2) Keep within the tyre manufacutrers recommended tread width per wheel width
these provide two pieces of evidence that will support you in court - however..
3) ensure suspension changes do not result in a difference in clearance, camber to a degree it could be percieved as dangerous condition.
4) ensure suspension changes or wheel offset does not bring the wheel or tyre too close to the wheel arch, inner wheel tub or suspension components, as they can argue under extreme load the tolerance/space is not enough to allow full motion of wheel without contact
5) if sticking within the tyre manufacturers guidelines make sure it does not bring the tyre too close to the wheel arch/suspension - if this is unavoidable, you must opt for a narrower or higher offset wheel.
6) whilst excessive negative camber itself is not an offense directly, it could be percieved to be putting the vehicle into dangerous condition due to the load against suspension components like tie-rods, cv joints etc.; along with the obvious issue of causing uneven tyre wear.
7) ensure vehicle is not excessively low - as again, any contact between the vehicle and the road under load; or any contact between wheel and tub can be percieved as dangerous.
8) ensure front wheels can turn lock-to-lock and still maintain sufficient clearance all round - if your only solution is stretching the tyres - correct the wheel offset/width.
He also advised:
1) Neither an MOT tester or tyre fitter has any influence; it's not their job to ascertain road legality, just road-worthiness - they are not one-in-the-same. As of 2018 an MOT tester can no longer fail a stretched tyre according to Vosa but that doesn't make it legal.
2) We're not out to just collect revenue or meet targets, or to stop people modifying their cars - We're the guys who have to go out and see people in horrific states and in pieces following road accidents; and whilst people are often sure of their knowledge and mechanical abilities; when it comes to an emergency driving situation; the vehicle won't behave the same - and it's undoubtadly safer to stay as close to the manufacutrers design as possible in this instance. We only want people to be safe and not to endanger other road users.
Now I can say from personal experience that I've had the odd dick-head cop do me for the most minor of things, and have a sarcastic attitude to boot. BUT I've had more pulls where the police have been very plesant, decent and fair and I think that's mostly down to my friendly attitude when they come to the door. My last pull for stretched tyres and excessive lowness resulted in an hour of getting the craic. So maybe instead of viewing as a competition to see who can get away with the most - why not just be a little more sensible when modifying cars? Instagram lows aint much use for daily driving anyway lol.
