I know the mains drifts in frequency, but I'm sceptical about a couple of things and I wondered if
Does it really drift enough within a typical length of a recording for you to be able to fingerprint it from the frequency history?
Is the frequency totally constant across the UK grid?
If this is on the level, then hats off to them, I'm very impressed, and also surprised that they've publicised it. Note to future kidnappers — make your ransom tape outdoors on a battery operated device!