I enjoy your sarcasm, but I will still answer your 1st question as if you were serious.
How do you know it's not a case so important and transcendental that absolute secrecy is required to protect British society as a whole?
Because the system on which our liberty and freedom is based is more important than some guys setting of a bomb, no matter how large the attack.
We just cannot - under any circumstance - accept a situation that a government can capture, try and imprison people without ever having to be accountable for those actions.
I could accept a situation where trial is postponed because of ongoing investigations against others, but the trial must be public. Heck, we (= the West) have been fighting regimes that did this in the past, saying we had to liberate the people from the oppression, etc. etc., and now we're doing it ourselves? Does not compute.