IO540, as you no doubt know using your mobile whilst airborne is also illegal, but for much better reasons than PCL being so, which may well seem no more than pointless Blinkered British Bureaucratic Bollix. (and its got nothing to do with messing with your nav kit either)
Hotel Mama and others, the "truth" is very simple. The VHF a/g frequencies are allocated (by whoever allocates the electromagnetic spectrum, ie not the CAA) for voice comms only. PCL is signalling which (surprise!) is a different classification to voice, ergo; illegal. Anywhere else such distinctions might be ignored, but you ain't anywhere else, so it sticks. Elsewhere people tend to be sufficiently responsible to use such things correctly, here we seem to have a way of taking the p!ss out of such concessions. It wouldn't be long before some berk started using PCL for starting his central heating or sending burst transmissions to a fax machine, so the bureaucratic mind just says, "No!".