Is my contractor's proposal for a driveway base in Scotland suitable?
The most important step is the "base". Here, we excavate down about 12", then compact the existing ground (your finished product is only as good as your base), then install 10" of crushed gravel (not round rock) that is evenly graded from 2"" down to fines (sand), then install a leveling course of 3/4" crushed rock that is evenly graded down to fines and then place the "wear" course on top.  (Of course each layer is compacted as it is installed.) The wear course can be asphaltic paving, concrete slab, reinforced concrete slab, stone slabs, brick pavers, etc. 

Compaction can be by manual tamping, but it's better with a mechanical tamper. 

We use crushed rock for the base because it "locks" together and is cheaper than making the wear course extra thick. 

All driving surfaces will settle. The key is to break it up so the cracks are not noticed. We do this by using slabs with "control joints". so when each slab settles, the cracks are small and "designed" into the surface. 

You  mentioned installing "slabs". So, if you use concrete slab (with or without reinforcing) you need to divide it into "slabs" with control joints every 15' on center each way for non-reinforced concrete slabs and 25' on center for reinforced slabs. (Be sure to stop the reinforcing from going through the joints too.) The reinforced slab is good because it will "span" soft spots and not have differential settlement, but the reinforcing and location of the control joints need to be designed by a professional. 

If you use smaller slabs, maybe something that could be picked up by 1-2 people, then each slab will move, twist, etc., but not be held in line and show more differential settlement. 

If you're just building a couple parking stalls, I'd have my contractor install the gravel base and compact it and then stop and let you park on it for a couple weeks. This will take most of the settlement out. Then, the contractor could spread a thin layer of gravel to level it all up and then install the wear course. 

What we use is not necessarily what you should use, because it may not be available without great cost. However, the principles are the same: 1) compacted soil, 2) good rock base, and 3) well placed sound wear course. 

Make sure you have good drainage and your drive/slab will be there longer than you...maybe as long as The Appian Way.