This seems to be a line that crops up a lot in discussions about guns.

Making guns harder to obtain, and it being a red flag if someone even owns one makes it much harder to plan such an attack, this is why the USA have a lot more shootings such as this one than other countries. British criminals still commit crime, and a few do have guns too, but the rarity of guns and how difficult they are to obtain reduces gun crime.

Another thing to bear in mind on this line of argument is that the people carrying out these attacks are unlikely to be hardened criminals or part of a highly organised groups, i.e. they are unlikely to have a network of connections that can easily get them illegal items without being detected; so guns being illegal would make it much more difficult for this kind of attack to happen. However, if you can simply go and buy a gun and a lot of ammunition then barely any premeditation or coordination is required for one of these attacks.