What's critical is how wide the sheet of rock is - if it was, say, only a foot wide, then you'd stand a chance of planting a hedge -if, though, its bedrock and extends a fair distance, say more than 6 feet either side, then drainage is going to be a problem for any plant trying to grow there, not to mention insufficient depth for the roots for any larger plant. It might, in the end, be better to erect a fence of some kind.

Silver Birch Trees grow on hill sides with very thin soil.
They also seed freely. You could probably get plenty
for free, with permission from the landowner.

What hedging is growing naturally round about?

Would a six foot high beech hedge be likely to grow in a 12 inch deep soil over an extended bedrock.

No.

I planted a Beech hedge on clay soil, lost 80% of the
plants. Then Monty said Beech will seldom grow on clay
soil.
You could always keep a Silver Birch hedge trimmed.
Someone once said its trunks always show up in
headlights on foggy nights.

Just wondering if Lonicera nitida Baggesons Gold would grow in these inhospitable conditions. It seems well nigh indestructible and its roots would grow sideways. The only problem I can see is if the site is exceptionally windy.

Might stand wind if its clipped closely and not to tall- good idea Wendy.
Try it with one first and see if it seems happy, so as not to waste a lot of money in case it doesn't work after all. It would need lots of watering until very well established though But trying to grow anything six feet tall on that depth of soil wouldn't work.

Carpinus, (hornbeam) will stand up to drought conditions rather than beech, but I would go with the fence idea as any shrub (apart from bramble) that I can think of would die eventually or be stunted.

You don't say how high you want the hedge to be. People are assuming you want something in the region of 5-6 ft high.
There is a suggestion to look at what is growing in the area - what do your neighbours have? I was thinking of gorse/broom and other plants of upland heathlands. Think windswept moors!

Gorse is the very devil to trim though, and it does seed around. But the trimmings are brilliant for getting a bonfire going...I wonder if blackthorn would survive either, its pretty tough. But whatever you choose I'd just start with one or two to see what happens. Even erecting a fence will be difficult if there is bedrock only a foot down. Perhaps a post and rail fence with brambles? Probably better than a solid one as less wind resistance so probably shallower footings would be OK.

Davecarr, what do you think so far???

Thanks for all the suggestions, folks. I think I might just try a mixed boundary with roses, and other bushes. Just want some visual divider of my land from the road.

You would need at least 30ins of soil to grow roses successfully - cheapest option would be to buy some native mixed bare root hedging - that way you could see what would grow without wasting a lot of money. Google bare root hedging and you will find loads of suppliers.