Upright fruit / nut tree for hedge

Best chance of success would be what you're already contemplating: pears on Pyrus (seedling) rootstock or apples on M25 rootstock.
I'd also look for a variety which is vigorous and/or not precocious because in my experience, varieties which are precocious tend to give up growing in tough conditions.

Pears are always tricky to recommend because they are not as widely available, tend to get caught by frosts or flower when there are no bees around - and everyone grows Conference so its disease resistance is gradually failing.

Apples could be more interesting.

If I had to pick a 'dirty dozen', grown on M25 rootstock, with varieties chosen for their ability to put up a tough fight against other established plants, they'd be:

Ashmead's Kernel
Belle de Boskoop
Blenheim Orange
Bloody Ploughman
Bramley's Seedling
Coul Blush
Edward VII
Gascoyne's Scarlet
Gravenstein
Hambledon Deux Ans
Laxton's Superb
Tower of Glamis

In many cases their growth rate is nothing special, but their toughness and adaptability allows them to keep growing while faster-growing but less-rugged and less-adaptable varieties struggle. I grow several of the above because my soil is dry and infertile. Other people in my region appear to be having great success with the varieties on the list which I don't currently grow.

If I could choose only one, it would be Gravenstein on M25.

I would also suggest an approach of no feeding and no watering (except if the tree wilts) because otherwise you'll encourage the hedge roots to smother the tree roots and you'll encourage the tree not to bother sending out roots to find its own water and nutrients.

Hazel?

You can cut it down to the base occasionally too ....as in coppicing.

Hazels are at least as strong-growing as the strongest fruit trees. Cosford is one of the most vigorous named varieties but it's more a catkin producer than a flower/nut producer.
Most hazels are on their own roots so there's no problems with coppicing. Hazels can live for centuries if coppiced every several years to regenerate and reinvigorate them.
The wood is useful for various things due to it being very bendy/flexible.

What about upright growth? Of course I could prune both apoles and hazels upright, but apples at least tend to be more spreading without interference, which was why I wasthinking pears might be better. Are any of the mentioned varieties very upright, at least when young?

For apples, Kidd's Orange Red grows with a strongly vertical habit.

I've now ordered a Cockle Pippin on M25. Everyone seems to agree it has an upright habit and is a healthy tree, so hopefully I can encourage it to grow into the right shape.

Does anyone have a Cockle Pippin?

Nope, but I've just looked it up
Sounds tasty , and a good storer too!