... why would they need to? The security service only has to ask a minister for authority and they get it, and then the intercept is legal.
The problem is that if intercept evidence is admissible, the defence can demand to see it whether or not the prosecution want to use it. Even if there are public policy reasons not to admit it, the defence will be aware that the evidence exists, and can use the fact that it is not admitted to persuade a jury that it shows the defendants are innocent, regardless of whether it actually does.
As to whether the methods and equipment used in the UK are already in the public domain, how would we know? The whole point is that at the moment that information is not released, so they could have publically unknown methods, or just not want the public to know which particular methods they do use.
Holding in custody for over 28 days is a power that the police claim they need, but on their own admission, they have never encountered an instance in a terrorist case where they would have needed to use it.
What I find even worse though, is the suggestion that questionning of suspects generally will be allowed after charge. It doesn't take much imagination to work out how the police will use that power. Without enough evidence to charge a suspect, they know they will have to persuade a court that they can hold on to him any longer unless he's charged. Charge him anyway, and keep putting the pressure on him afterwards, when there is no limit on how long he can be held and worked over, and the court will not be overseeing it.