So much firearm law requires people to be trusted to stick to it all.
Take FAC barrel length. It would take but a minute to hacksaw a barrel so short it would become a Section 5. That makes every gun "capable with hacksaw".
UK Law requires a certain power for the bigger deer species, so a .243 almost definitely requires a 100 grain plus bullet even though the calibre was never really intended to shoot such heavy bullets; its much happier with something lighter. Don't do it.
A humane dispatch handgun may well be authorised with a restricted magazine. Magazines are not licensed so anyone can own a factory high capacity one. The conditions are broken if a factory high capacity loaded one is fitted into the well. Don't do it.

Same with that washer. Leave it alone; don't go messing about.

There is a certain amount of trust with guns. Safety and sticking to the laws governing them. If you have good reason for more, just get the license for more.
If you want more than UK allows then move to France If more than that then USA , and if you want to save the world go and sign up in Ukraine.
In the UK we play cricket, not football.