Tereasa May has a hard on for a police state. This is just phase 1. In fact no, the snoopers charter was phase 1, fuck lot of good that did clearly. I would rather live under the threat of terrorism than sit back and watch civil liberties be eroded by a right wing totalitarian government for my "safety".
Theresa May has *nothing*& to do with this decision on the threat level. Nothing at all. It's done by an independent committee that she cannot influence, by design. Even a few hours ago she was talking about the level remaining at severe. It has been changed due to intelligence officials having solid intel on further attacks or an active cel.
That does not mean there will be an attack, for a variety of reasons - but this isn't political, it's purely about the risk of a threat. Critical has historically only ever been used for a few days at a time, because it is a threat and intel specific rating. You can't just push it to critical and leave it there.