It's certainly not illegal to inspect unattended items in the UK. An assessment is typically made of whether or not it should be deemed a threat.

If the item is not hidden, not obviously suspicious, and is typical of the environment in which it was found, then it's unlikely to be a threat and can be inspected further.

Unattended bags are found all the time. There's no need for the military to be called in for all (or even most) of them. And even then, that's mostly the responsibility of the police than the armed forces.