Well they technically do, parliamentary sovereignty means that the UK can, "legislate to ban smoking on the streets of Paris." There are no limits to what laws they can write, even ones that are out of their jurisdiction, absurd, or unenforceable.
Not only is this enforceable, it has been enforced, and people have been assassinated without charge for this crime.
The size of ones military expenditure does not determine whether a foreign government can kill you, specifically.