Hi Pete,
I think we will have to agree to disagree on some of this. I belong to a local bee-keepers association where some members keep bees in the old tradition, usually the older members, but there has been a surge of interest in the last couple of years and there are lots of younger people who are adopting more natural methods. The club apiaries were all destroyed last year due to EFB. These apiaries were well "managed" every two or three weeks throughout the summer months, so the theory that swarm control helps to prevent disease doesn't seem to hold sway. Incidentally, my hives are within distance of the club hives, which is why I was obliged to have mine inspected. Of course you are right that there is a risk of disease from a collected swarm, but bees risk this contamination from other bees all the time as they are foraging. I haven't yet had a swarm that was carrying a disease.
I don't know where you live either, but our members are very eager to get on the swarm list every year, and certainly don't see stray swarms as some kind of nuisance. Yes, the colonies do have different temperaments and if you are unfortunate enough to have a really aggressive lot, you could change the queen. The only really aggressive bees I have encountered were those being sold complete with hives at a bee-keepers sale. They actually came across the field as you approached, to sting you, incredible!
I do think that traditional bee-keeping has played a part in the decline of bees, although I would lay most of the blame with the use of pesticides and monoculture. Maybe that's why the bees do well in London.
Just as a point of interest, I was called to pick up a swarm which had come from a huge barn. I hived them at home with the intention of moving them up with the others if they were ok. They swarmed the next day at noon into a plum tree. I knocked them down and put them back in the hive. The next day they swarmed again to the same tree, and I knew they weren't going to stay. I had to go out that afternoon and so said goodbye to them. They obviously didn't like the hive.