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hoefla-
A (garden centre bought) blackberry should in addition to being a deterrent to intruders, give large, tasty fruit from strong thick shoots, which will also encourage some wildlife.
A bramble may give some fruit, but more likely will give tiny dessicated unsatisfying fruits*.*Brambles do constantly hybridise, so you may get lucky and end up with a bramble that was pollinated from a choice fruiting blackberry.
mespilus
The best allowable anti climb/deterrent is a blackberry bush. Thread the growing shoots through each gap to present a continuous thorny hedge