Choosing native hedging for a small urban garden

Hi Evv Euonymous Fortunei Emerald Gaiety (with the silver queen one) would work as can be pruned. For your sizes - Lavendula makes a good hedge. Boxus can be cheap!
I would avoid prickly plants in this area and on the plants the spacing is usually given.
Good luck

I have blackthorn and hawthorn down one side and it is lovely and as you say they are british natives. You can prune as hard or light as you like. and the thorns are a good deterent for people walking through.

the flowers are good for the insects and then berries in the autumn for the birds too. only draw back [ if it is one] is they are both deciduous.

You might be better using a climber on the fence by the path instead of a hedge, plants always spread further sideways than you think. You don't want to rub against wet foliage all winter. Euonymous could work as long as you clip it regularly to keep it within bounds. Cornus would be good for the other side as you cut it down to 6 inches every Spring so size is not such a problem. Elder gets huge and you will regret it. Shrubs will need to be 4 or 5 feet apart otherwise they will crowd each other in a few years. Plant herbaceous plants in between whilst the shrubs grow.

If you're looking to create a hedge, recommended planting distance (depending on variety) is a foot to 18 inches apart. For the area you describe as being only 50cm deep, I think you might be better off looking for plants which have a columnar growth habit - everything else will not like being allowed to only bush out by 18 inches. If you don't mind a deciduous hedge, there is a columnar dark leaved Berberis - thunbergii "Helmond's Pillar". There are also some conifer cultivars which are naturally columnar in shape, though you'd have to select one that didn't mind being trimmed regularly. The one you've mentioned that will tolerate being cut and restrained back to 18 inches wide is Ribes sanguinum (flowering currant), though having to restrain it so much might mean you lose flowers. Otherwise, Euonymus might be the answer, as long as you're prepared to keep on top of restraining its spread.

Thorny hedges leave thorny clippings that you might miss (think of kids, pets). If its a perimiter hedge and you want to keep out tresspassers then thorny is the way to go.
If the hedge will be next to a footpath, do you really want to snag your clothes every time you walk past? Plus Hawthorn and Blackthorn are slow growers. Same with Box (Buxus).
Do you want an evergreen or a deciduous hedge?
Chinese Privet - evergreen.
Beech - Deciduous (but it holds its dead leaves overwinter).
So much to think about, so much choice.