UK police are mostly not armed with a gun. The crime levels do not facilitate this and neither public or police want a fully armed UK police force, although both support more trained firearms officers and more armed units. Most are armed with a 50000v taser, baton and cs spray. Body armour also.
Inaction isn’t exclusive to the British. That part of London sadly is latte country. Had he tried it in other parts of the city and UK, he’d have been beaten. Remember Glasgow Airport.
Had he tried here, in a free state, he would have been shot.