the right way to do your driveway is dependent on a few things, but primarily, its water and cold. in scotland, areas down by the coast will have more rain, but less cold. inland or in the highlands, you will have less rain (and snow) but more cold, and thus more frost penetration.
i have seen this problem here in southern ontario for decades. its almost always entirely based on inexperienced contractors putting in things that they don't really understand. remember, for many contractors, their warranty is only as long as it takes for your cheque to clear the bank.
when you construct a driveway like you are describing, you have three issues that create the ruts you don't want.
first is the soil fluidity below the driveway. water, freeze/thaw cycling and vertical load all cause the soil to subside in similar ways such that areas of high load (where the tires are) to sink more than other areas. this is why the whole thing is dug out and replaced with coarse gravel. it helps to allow drainage (no water means less fluidity to the soil), it helps to spread the load out (the tires push down on a small contact area, but the gravel spreads the load horizontally through the base proportionally to the depth of the gravel bed)
second is the amount of precipitation you receive and how much it freezes. the more rain you get, the more "lubricant" there is in the soil, gravel and between the paving stones. this all contributes to things moving, which is what is makes the ruts. if you set the pavers into sand only, it maximizes drainage, but lets water get under the driveway. if its not properly sloped or drained, that water will lubricate and under cold conditions, freeze. freezing is the biggest destroyer of driveways as the mechanical pressures from ice formation are huge and can move things large distances. if you put the driveway on concrete (as your italian friend suggests), it has to be well drained and well designed/reinforced. it will keep the water out better, but over time as small fractures and lack of sealing let water in more and more, you will get more water permeation. but now it has nowhere to drain to (as the concrete isn't porous the same way gravel is) and it will cause more damage if it freezes than it would with just gravel. this approach is fine in warm areas like Mediterranean italy or southern california, but its very expensive to do it properly in cold or cold and wet areas like scotland.
thirdly is mechanical reinforcement. this is an often omitted step that lots of less qualified contractors omit. by adding metal, or preferably fibre-reinforced polymeric binding fabrics, in the right ways and in multiple layers, you can horizontally bind the base aggregates. this may seem counterintuitive, but binding the base horizontally almost entirely eliminates the rutting problem you want to avoid. since it prevents the base from moving on a microscopic scale, it also prevents vertical movement of particulates. which means no ruts. however, it has to be done well and with the right geotextiles. its not just weed fabric from the local gardening store. someone with experience with these systems in your specific area will be the best choice as they will know the available fabrics and how they interact with locally sourced aggregates.
a really simple solution is just to hire an independent engineer to design the parking area. they will specify exactly what to build and how to do it. then any knucklehead landscaper can install the parts. you'll get a better slab, they will learn the right way to do it, and everyone is happy. might cost you a couple thousand bucks extra, but its worth it if the slab stays flat for 30 years.
