Britain legally allows shotguns, rifles. But not assault rifles. They were banned in 1988 after Michael Ryan went nuts with several AR’s in Hungerford in 1987.
Most UK gun owners (my family inc) never owned one and those that did were Rambo wannabes. UK gun owners were more concerned with laws regarding handguns. Even from UK gun owners (maybe wrongly in hindsight) there wasnt a great deal of opposition to the 1988 ban, many thought the Rambo wannabes were giving responsible owners a bad name.
I am not sure what the laws are regarding an American owning guns/importing their own guns into the UK. You’d certainly need a licence and safe stockage of the weapons. Its the importation of own guns I am not sure of.
“But not assault rifles.”
Pity. When you need one you won’t have one.