I seem to recall (and it is a long time ago since I got my licence) that the confidentiality aspect was to do with listening in to potentially sensitive information being passed when purple airways (royal flights) were active. That seems so ludicrous now that I'm sure someone will be along shortly to explain the correct reason.
However, the Wireless Telegraphy Act is quite clear, before you can transmit or receive on an airband frequency in the UK, you must pass a test and obtain a licence. Normally this is done at the early stages of flying training.
The reality of course, is no one bothers for receive only instruments. I can't imagine a situation where a prime prosecution would be brought for the offence although if one were to have "the book thrown" for some infraction caused whilst listening then maybe it would be added.
It may well be in these days of terrorists around every corner etc, that some eager official would decide that you listening to your pilots every word breaks some security rule somewhere.
I would also add, that to listen to your particular flight without a scanner would require you to know the ground, tower, approach and transit frequencies along your route. However, you wouldn't be able to listen to ground, tower or (much of, if any) approach as they would all be active during the ascent/descent phase.
Personally, I'd watch the film and down a couple of large G&T's.