If the police are that bothered by them I would get a written statement from them along with their contact details and take it to Tesla and tell them to sort it out and if they believe all is fine, here are the police's contact details and they can talk between themselves. Given you've done nothing wrong here I wouldn't be at all happy receiving even a caution.
My experience was this:
The police officer was looking over the car, in a car park, seemingly just because he'd not seen one before and was clearly interested in it, as he asked the usual questions about performance, range etc. When he spotted the misted up real lights, he warned me that they were defective, and that I needed to get them fixed immediately, or else risk getting a fixed penalty for driving with defective rear lights. He was very polite about it, and when I explained that Tesla considered this to be just cosmetic, he replied with words to the effect of
"Tell Tesla the police consider them defective, and will prosecute the drivers of cars with lights like this". He was very friendly about it, and I'm sure his intention was to give me leverage to get Tesla to sort them (they had refused to fix them on a previous SC visit). This worked, when I told Tesla that the police had ordered me to get them fixed, they agreed to replace them
The law seems clear, though. It will not be the car manufacturer that gets penalised for something like this, it will be the driver of the car (not the owner, either). It's the driver's responsibility to ensure the vehicle complies with the C&U regs at all times when being driven on the road, to the best of their ability. It would be hard for a driver to argue that partially obscured rear lights weren't something that could have been easily checked before setting off, so probably hard to get any penalty quashed, I think.
We're all supposed to check our cars are roadworthy before we drive them, for every single journey. I know that very few people do in practice, but that's what the law expects. Back when I was working, and we had pool cars for staff use, this inspection procedure was rigidly adhered to. The first person signing a car out in the morning had to give it a full DI (daily inspection) with a check list (tyre condition and pressures, lights all functioning, windscreen wipers OK, washer bottle, radiator, oil level, etc OK). Subsequent journeys were recorded line by line on a chit in the car, with a signature to say that the vehicle had been checked before that trip. It seems OTT, but it is what the law expects us to do as drivers, and my employer had just, rather pedantically, put that into a written procedure, most probably to cover their backside.
Anyway, the main issue here is that there is a recurring problem with the rear lights becoming partially obscured by water ingress, and whether Tesla believe this is acceptable or not makes not a jot of difference when it comes to the hassle a driver may have if he/she encounters a particularly awkward police officer. Arguing that it's Tesla's fault won't wash with the police at all, although it may result in an outcome like mine, with the police officer lending weight to my argument with Tesla about this if someone's lucky. If someone's unlucky, perhaps because they've encountered a particularly awkward police officer, then they could easily get a fixed penalty, and have the hassle of trying to argue their way out of it.