I just want to point out a couple of things for those people who are really panicked by this. By the time I write this others may have said so apologies if I am repeating.
-The nature of the way we run our security services means that the PM etc can't just decide to put armed forces on the streets. We are not a dictatorship.
-On a practical level if we want to tighten security we need stuff like trained people and money.
-Due to the above facts we need a plan in place for if we need to bolster security.
-Those plans are well developed and allow services to act in a way that they normally wouldn't be able to. They aren't back of an envelope things, part of these plans include the threat level.
-"Critical" is an abstract thing, whatever the official definition is it is ultimately decided by a group of people who are looking at things like resources and how they are going to be able to put extra security in places on a practical level.
-I don't know what they know (obvs) but it's worth remembering that these measures are practical.
Sorry for the bullet points but I am shit at putting a lot of information in paragraphs.