Unfortunately the law in this area is very grey and almost universally ignored. Otherwise the police/ radiocommunications agency would be rounding up spotters and confiscating their equipment in the same manner as CB fans who had illegal AM sets were prosecuted in the early 80's. Even now it's best not to draw too much attention to yourself if you want to listen in (especially when abroad!) despite being a relatively harmless hobby. In fact many pilots/ATCOs have been introduced to the industry via the monitoring of the airband frequencies.