Berberis will make a good sharp reminder to keep out
I did wonder about planting the verge with something like berberis and gorse but it's about 100 yds and I'd need a lot of plants, plus it's been rough for years so would be difficult to dig. Don't think I can afford it.
Bide your time buy three or six "Jennifer " berberris they are the spikes from hell and grow them for cuttings , , tear them off a less than 2 yr old plant , use a hormone compound and grow your own clones for a couple of years , set the growing bed over some well cultivated ground that has plenty of rotted muck in it and cover the bed in black poly sheet that has the edges weighted /dug into the ground this serves not only weed control but keeps thing moist and channels rain to each plant . at each cutting site make a small heel dent in the grond and put a small slit in , bung the cutting in ..set them out when 20 inches high in early spring along a Round up sprayed rotavated bed just outside the foot path bounbdaries and run a wire along the line on waist high posts .
Set them in slightlya staggered formation one every 2 feet .and use a general fertilizer to bring them on .
I did this to thousands of cuttings from all sorts of plants... Laurel works well for a solid wall of impenetratable growth ( can grow to 30 ish feet ) to the average punter when propagated properly.
It took me four years to make a 200 mtr long wall of it five feet high laurel .. but is not so good in stock places . My mate Don actually wove the young four foot plants like a laid staked hedge without the cutting of the wood to make a totally impenetrable wall of it on his small holding .