Dfyjkt

A friend of mine got fined for "belastingontduiking" (tax evasion) driving in NL with a BE car, owned by his parents who live across the border, in Belgium, 5 km away from him. This resulted in this question, as my husband and I will own a Dutch drivers license and a British car.

Assume someone owning a Dutch drivers license and a car with British license plates living in the UK. This is allowed. This person wants to visit the Netherlands for less than 90 days (beyond that they have to register as a resident) and takes their British car and Dutch drivers license to Netherlands. Can they be fined for doing this? If so, can they take this to court and win? If yes, how?

If it matters, I am Dutch/Belgian, husband is British.

Inside the EU, the country of issue of your driving license should be irrelevant. What is important is that the registration of the car matches your country of residence. And even so, it only really matters in your own country of residence ; you can go on holiday to Italy from Sweden and rent a car there.

The police officer may assume that you are doing something irregular to evade taxes, but if you show the tunnel/ferry ticket with a reasonably recent date and some proof of residence, it's hard to imagine how they can try to push on that.

Your friend's fine was near a border, where people pull all sorts of tricks to evade taxes, and authorities tend to have a harder hand. A British car in the Netherlands is "two countries away" and in the wrong side of the road to be suspicious.

To accomodate the people living in/around Baarle-Nassau, there's bound to be some special rules between Belgium and the Netherlands, but I don't know them - but in general the rule is that a car should be registered in the country where it is used. The point is that taxes associated with the car are meant to contribute to the society that has expenses (e.g. maintainance of the roads) from it's use.

Of course there are rules for passing through and vacationing in other countries, but as soon your use doesn't match those, the rules get strict. An example:
I live and work in Copenhagen, but have colleagues who live in Malmø, Sweden. They are allowed to drive their Swedish registered cars to/from work, but (as far as I have understood) they are not allowed to use it for trips wholely within Denmark, i.e. they can go shopping in Danish stores on their way home from work.

When you have to stay in a different country for a limited amount of time there's a possibility to get permission for using your car while there. When I was 9 (so too young to care about the tax details), we lived in France for ~3 months, because my father had to attend some course together with some of his colleagues, some of which brought and used their Danish registered cars (my family didn't have a car) - they probably paid some tax to France (and possibly got some tax return in Denmark).

Returning to your cases.

The Dutch authorities were probably right when they issued that fine, and you should look into if your planned stay in the Netherlands is so long that you need a permission to bring and use your car while there.

What matters is the country of residence. You did not write that explicitely but it sounds as if your friend might have been a Dutch resident (possibly "borrowing" his or her parents' car regularly). I have heard similar stories about German citizens driving a German car with a German driving license (not even close to the border), the issue is that the Netherlands has quite high taxes for cars so people are tempted to circumvent them by using a car registered abroad long-term.

So, in principle, what you need to have is some evidence that you reside in the UK (although even that might not be needed as it should be easy for the police to check whether you are registered as a resident in the Netherlands).