Re: Not the story it first appears to be...
I don't see the problem either; it's no different to subpoenaing phone records, recordings of conversation or obtaining any other evidence as relates to the case.
There is a necessity to prove or have agreed that whatever is subpoenaed is actually admissible and evidence but that's no different to any electronic, hearsay or circumstantial evidence.
"Evidence is evidence" as far as the courts in the UK see things though it is different in other jurisdictions. We don't have such American nonsense that a gun used in a crime found in the defendant's dustbin cannot be presented as evidence because there was no warrant to look in the bin.