This is correct, but anyone in England has the right to use such force as is reasonable in the prevention of crime or in the apprehension of an offender. This includes lethal force i.e. killing.
I would think the police are off the hook. And so would I be, if I mistakenly but honestly killed someone whom I thought was about to kill me or a third party.
These rules are quite reasonable in themselves. So, although it is correct to say that the police have no special right to kill, the point is that were I or you in their shoes, knowing what they thought they knew (the last point is an hypothesis. I don't know what they thought they knew.), we would have the right to shot dead the suspected bomber.
An English lawyer.