'Could be' and 'are' are two different things.
Where's your actual evidence they are illegal? You don't have any.
What reason do you have to assume this procedure is illegal? Other than some kind of irrelevant association with the surveillance program of a different part of the government proving to be illegal.
I'll gladly accept the procedure is illegal provided with proof it is, so... over to you.
You are stating that it is legal, and from my point of view your only reason is that the accused said they were following procedure. As following procedure doesn't show legality, why are you certain it is legal?
If it were illegal it wouldn't be in place, full stop. This is the crown prosecution service, what ever you may or may not think of them, their procedures have to be legal. If a procedure that is in place is challenged in a court and found by the court to be illegal then fine, it gets changed. But those following said procedure beforehand are not breaking the law and are acting in good faith.
But data retention isset in law (at the time under the 1998 act, and now the gdpr and 2018 act) and the cps follows those laws like everyone else must. Their procedures follow these laws.