Everything's private by someone, council land is normally a free for all so I was only speaking about private land. As I say go look it up rather than taking it from me but yes, you are allowed to go walking through any fields, the restrictions are mostly based on if there are sheep lambing or if there's a residence on the bit you're going through so a courtyard would be out of bounds but the field around it wouldn't.
As this is a barn on paper there's no reason you can't go round it as close as you want, going in it would have the usual trespass issues but nothing illegal.
The farmers gun license will be for vermin only and he will only be allowed certain shot. If he has no license or he uses it in any other way than to control vermin then he's breaking the law and will be arrested, if he's charged depends on how much noise you make to the police and if they can be bothered. He doesn't even have to point it at you if he's holding a gun and threatens to shoot you or even just holding it and you are worried he may use it are all arrestable instances.
Dogs are classed as a dangerous weapon, no one is allowed to set a dog off its lead to attack you, even the police have to justify themselves doing it on a case by case thing so its tightly regulated. Threatening to or indeed letting any animal off a leash or out of a confined area in order to chase or attack someone is a criminal offence.
That all said, people normally just do one when confronted with a farmers gun as they don't want to tempt fate but i personally had a recent issue of a shotgun being pulled out of a landrover to which I let the guy know without mincing any words where the gun will get shoved if it wasn't put straight back in the vehicle, the gun got put back and I carried on my passage.
The guy at Denbigh got shut down eventually and the guy shooting at people trying to visit the underwater ballroom got jailed last year so you better believe the one with the gun is in the wrong. There's no instance in the uk where pointing a gun at someone is ok in the eyes of the law.