"Having now got everything down to ground level it's obvious that the bramble & nettle infestation has impoverished the (light/sandy) soil to the extent that there's a lot of moss (now dry) cover over a large area and little evidence of grass.
"Do we just wait for the rain to see what appears?"
Firstly, well done Martin! If you can see bare soil, then you have done a proper job!
As for the mossy, impoverished soil, that will be because brambles are greedy swines, and will also have shaded the ground, which promotes moss formation.
However, all is not lost: nettles have a reputation for being able to access nutrients well below the surface of the soil, with their long yellow roots. They are often used as an indicator that the underlying soil is quite good, even though it might look quite poor on top.
So, what would I suggest? Bearing in mind that I haven't seen it... I would suggest raking off the moss - it does no good, it doesn't rot down and enrich the soil in any way, and it will be full of moss spores, so if you leave it, you will just get more moss.
So if it's all nice and dry, rake it together into big heaps and burn it!! OK, you could choose to bag it up and bin it, or take it down the tip, but it is usually very bulky stuff, so it takes up a lot of room in the wheelie bin or in your car. If you burn it, it destroys the spores, saves you a trip to the dump, and you can rake the ashes out over the soil, where any nutrients will be given back. Hooray!
What will that leave? An area of light, sandy soil, with very little growing on it. (And possibly one scorched patch where you had the bonfire, but don't worry about that...)
Perfect for making a meadow!
When people try to create a wildflower meadow from a grassy area, the usual problem is that the soil is too rich, and the grass is too dense and too strong, so wildflower seeds - and I assume, Martin, that when you say "meadow" you mean "wildflower meadow" - struggle to gain a foothold.
This means that if you have bare, impoverished, sandy soil with a sparse amount of grass, it should be perfect for scattering seed and getting a lovely meadow going.
As it has been unseasonably dry and sunny, with no end in sight, I would hold off scattering your wildflower seed until it rains, because if you do it while the soil is bone dry, the little birdies will come and scoff the lot. And the small mammals will grab any which the birds miss.
The act of raking off the moss will "fluff up" the top surface, which is probably pretty dusty and light anyway, so if you wait until it rains, or is just about to rain, the rain will push the seeds down into the soil, and they will germinate quickly: and once they have germinated, the small plants will be less desirable for hungry birds.
Hope this helps!