It's true, you're right. Very little is done for fish if anything at all. I do know the law has been used to prevent the sale of Koi at reputable shows organised by the BKKS but not to prevent goldfish as prizes!!! It's a strange world.
If 5" is the maximum you are looking for then koi aren't the best option, they could easily double that in just one year when small. Shubunkins are probably a better option as they look quite similar but won't grow so quickly or as large.
Grade A are often seen in garden centres and similar places. The term doesn't mean anything but it implies the fish are of good quality when quite often they aren't even a recognisable type.
Even though fish are given the same protection in law as all other animals prosecutions are expensive to bring so the RSPCA needs maximum publicity to get value for money. Prosecuting someone unknown for keeping a goldfish in a bowl will get zero publicity. Not only that but it is unlikely to succeed because the makers of the bowl will present a very professional defence full of pseudo-science which demonstrates just how their product is fine for a goldfish. In other words it will never happen even though we all know it is less than ideal.
There are a few locally but they sell the meaningless 'Grade A' Koi which are essentially the very poorest of quality. There is a dealer at Radcliffe Nr Bury which isn't to far away and deals in much higher quality Japanese Koi. I live very close to Blackburn and I travel to L.E Koi in Nottingham for mine, it's a good excuse to make a day of it and visit Wharf Aquatics too because they are only about 10 miles apart.
Some individuals are more prone to it than others, again with fancy goldfish they are all slightly different a on the inside as well as on the outside. Some will have slightly more out of shape organs than do others due to the artificially induced rounded body shape so yes genetics does have a part to play when fancy goldfish are affected and although I haven't come across it yet I imagine the newish trend of balloon fish will probably be more prone to this too.
Dried food is a known cause especially if it fed exclusively and flake food is normally fed at the surface which is again a known cause.
The dangerous wild animals act definitely needs to be looked at again.
It seems odd that small rear fanged vine snakes which really could be regarded as harmless due to their mild venom (less painful than a nettle sting) are on the list but you can legitimately buy an anaconda which easily grow to be large enough to be a threat even to an adult or any other large constrictor. Venomous fish including stone fish which I have seen for sale a couple of times I've also seen Hydrocynus goliath for sale recently although I have yet to see a privately owned aquarium which could house one.
Fish don't get swim bladder problems from food. They do have digestive problems where trapped gas causes a buoyancy problem but it is nothing to do with the swim bladder. A swim bladder disorder is entirely different and has nothing what so ever to do with diet.
Fancy goldfish are particularly prone to trapped wind, a diet to ow in fibre, to much dried food and feeding at the surface can all add to this problem. If a fish is affected by this then adding a teaspoon of epsom salt per 18 litres to a bath and keeping the fish in there preferably at a slightly raised temp (around 5C more than usual) will normally clear the problem within 24hrs. After that just avoid the common causes.
You mentioned in your first post that you thought the tail loss might be due to the fin rot that you are trying to treat at present. But now the loss is due to an attack by another fish which coincidentally attacked the very same are affected by the fin rot?
I promise you that this is fin rot and unless it is treated as such as mentioned earlier you will lose the fish.
I've included a clear photo of a koi with fin rot as an example of what an established fin rot infection looks like. It is of course entirely up to you how you proceed.
Take the plant out and just cut the stem, remove the lower leaves so that they don't rot in the substrate and then replant it. New roots will soon grow. Alternatively just cut the plant as above but leave the original in place where it is likely to grow two new shoots - increasing your stock for free.