Also in the UK and agree! Roughly equal split between ‘we were walking off leash and bam! disappeared’ and ‘dog was in the (secured) garden and has decided to leave home’.
My family have owned borders and I think HighTales dog #2 has been our dominant experience until the dog feels that another dog is a threat. (That threat can absolutely all be in their head). One of ours pinned a full size, fit, working lab to the floor by the throat and would’ve killed it if people hadn’t heard just because it was in the stables as a legit guest and Border didn’t approve. Another badly damaged another terriers leg because it dared to come up to my dad. Both dogs were leashed, in a public place, but still damage was done. Both lived within the home pack of around 3-4 dogs with no issues.
Characteristically they are bred to be brave and fierce. Their role was to flush foxes that had gone to ground (as well as generic ratting). So they had to be brave enough to take on a fox and torment it enough that it decides going above ground to where it knows a pack of hounds is, is the preferred option. Ours have been excellent ratters.
Long legs is ‘correct’ as the breed, shorter legs are normally crossed with JRTs. Quite a wiry frame and narrow chest is also correct. Sometimes you see those bred as pets as being quite barrel chested and unlikely to fit anywhere near a fox hole
So, in summary, lovely, smart, intelligent, people cuddlebugs and if you’ve got the right farm set up and can absolutely control who comes near - OR - you can really commit to a lot of socialisation, not just as puppies but critically at around 2.5-3 years (both of the aforementioned were dreamy with other dogs and well socialised until suddenly they matured to adults and weren’t. Incidentally, they weren’t related so it wasn’t a genetic temperament trait from parents).
Someone described theirs as jumping on and off the quad bike at will, which made me smile absolutely! So quick, agile and coordinated and determined to be with their human - but equally able to hop down and independently explore something that interests them. Describes them to a tee! Add to that their selective deafness and that they’ll only come back to the quad once they hear the engine moving and know you mean it - at which point a streak of black will hurtle up and on and nestle against you as if you hadn’t spent the last 5 minutes calling them!