I suspect that there is no clear, single law which states "Thou shalt not use a mobile phone in an aircraft" - it's more likely to be entrenched within some weasel-wording such as 'emitting devices used in aeroplanes shall be licensed and operated in accordance with the terms and conditions of that licence' or similar.
Anyway IO540, why not make yourself famous by ringing the CAA's Enforcement Branch from your spamcan and see how your test case goes....
