The Queen should declare the UK a shall issue state and make it stick. Her subjects are not allowed to defend themselves.
The Queen doesn’t have as much power as you think. Something that happened about 250 years ago might have had something to do with it.
After 1776, I would imagine the crown is a bit “gun shy” about guns in the hands of the ‘subjects’.
The Queen has the legal right to behead anyone she wants.
She should decree this bomber be beheaded in public and see what happens.