All trade association or professional bodies, I've belonged to, you have to agree to abide by the rules.
You may not agree with them, but you MUST obey them or you leave yourself open to be disciplined by them or in a more serious breach non-compliance would form a powerful case if you were prosecuted under health and safety laws or worse and in my view quite rightly so.
Normally regulations are drawn up by people with much experience in the relevant area with the use of accident and failure reports by the police, fire brigade and other interested parties. The regulations are drawn up and then circulated for comment from people within the industry, before they are enacted. Now if the regulations are found to be flawed, they go through the same process to produce a revised set of regulations and so on. Now by not complying this installer is ignoring all of this experience and IMHO is using a very arrogant I know best attitude, which I have seen before with disastrous results.
If it was me I would be asking him to provide MTBF (mean time between failure) figures for the use of cable clips compared to metal clips, when they are used in the environment you are using them in, with them subjected many different chemicals of salt, oil grease, anti-freeze, oil, diesel to name but a few and also hot and cold temperature cycling etc, etc. If he can't his experience is limited to that nobody has come back to him with a failure so far, not very reassuring is it.

Because all of these sorts of things have to be specified and tested by car manufactures, is why it costs over £1bn of R&D to produce a completely new model. What is his R&D budget and testing program for the bits that don't comply with the COP II regulations to make sure they are safe? My money would be on a budget of zero and his R&D is suck it and see.

Personally, I think now that you know it does not comply with COP II, it puts you in a very awkward position, unless you insist the regulations are complied with by the installer or you take it up with the regulatory body.
This is why:
Many serious and bad transport accidents are normally caused by a series of events. With the Titanic, it was the number of lifeboats reduced from that original design, the changed specification to cheaper inferior rivets, that were too brittle, a rudder that was too small, an officer with the crows nest binoculars key, leaving at the last minute the ship before it sailed from Southampton and taking the key with him, a fire in no 1 coal hold which weakened the bulkhead, where the coal was loaded too quickly and not damped down enough, if this bulkhead had held longer, several ships that arrived a few hours after she had sunk, may of got there in time, the officer on the bridge trying to avoid the iceberg (standard practice with mail ship), rather than hitting it straight on, him putting the engines in reverse, which stopped the central propeller, which provides thrust past the rudder, which makes the ship turn better, a radio message on ice not given to the captain, and an adjacent ship ignoring distress signals and their radio was turned off and of course a ship and iceberg in the wrong places at the wrong time. A lot of factors.
No image you do nothing and in a few years you sell on the car. When the new owner is driving a couple of cable ties break through brittle fracture, so the pipe is dragging along the ground, which causes a leak. There is a tank solenoid valve, to turn the gas off when the engine stops but it is faulty and doesn't seal properly. The new owner parks it in his garage overnight, so all the gas leaks out, into a vapour layer on the floor. The next day goes to the garage through the house to garage door, which automatically shuts behind him (building regulations mean you must have an automatic closure mechanism), The garage has no windows, which would act as a blast vent, so it is a contained space which will maximise the blast effect, his walking into the garage, kicks up the gas from the floor, he then turns on the light, which arcs and there is a big explosion. The explosion kills the new owner and injures members of his family. The garage side wall collapses into the neighbours garden, where a 5 year olds birthday party is being held. The wall collapses on them causing many causalities.
When it is being investigated, one of the children was related to an OOF member who recognized the instillation pictures shown in the press and passed this thread to them. Imagine when there is a knock on your door by the investigators how you would stand legally and how many nights sleep you would lose?
Now I have told you above and what I would personally do and why. It is up to you what you do and I very much hope that whatever you decide, it works out fine and you have no problems now or in the future. Good luck.