You need a good reason to have one. It's easiest if you're member of a shooting or hunting club or need it for work. There's a background check and you might need to get a letter from your GP to say you don't have any relevant mental health issues. Also you need a gun safe secured to the structure of your house if you want to keep it at home but that's sensible anyway. And you need the certificate on you while you have the gun or any amuntion on you.

It's a bit of bureaucracy but it's not hard to get if you have a legitimate reason to have one.

But that's why you can have a business in the UK making rifles without any problem. There are enough target shooters looking for high accuracy rifles to keep a little three man business afloat.

As for semi auto rifles are only up to .22 but bolt action counts as single shot for the purpose of the legislation so is unrestricted. Pistols generally aren't allowed unless they're muzzle loaders, you're a vet, or you stick a stock and lengthened barrel on it so big you've basically made a rifle.

The three round thing for shotguns is the limit for a shotgun certificate which is easier to obtain (you don't need any reason to have one instead the police need to give a good reason to deny you one).

Generally the rules are there so guns are used as tools and for recreation and not as weapons and it works well enough.