Basically they don't have to. Either they operate outside the law or there is obscure legislation that empowers them.

They certainly won't bother messing with anything that requires court permission..................mind you, when law enforcement and judicial appointments are political, things might be different?

Anyway who would need to when you have:

The Act states that emergency powers can only be used if an event or situation threatens: serious damage to human welfare in the UK, a devolved territory or region; serious damage to the environment of the UK, a devolved territory or region; or the security of the UK, from war or terrorism. They can be used if such a situation is occurring, has occurred or is about to occur.
And that is purely a government decision. Just how do you prove that you were not about to commit a terrorist act?

BTW that is the UK's Civil Contingencies Act 2004