It's nothing to do with the Official Secrets Act, but the Wireless Telegraphy Act. It is illegal in the UK to monitor any transmission for which you do not have authority. Aviation communications come within that category. It is only permissable to use the communications in the course of duty. Ie. Pilots and controllers are normally authorised to listen to aviation communications when they are not actually engaged in their duty.
Now, of course, everyone is going to talk about scanners, blah, blah, blah!