The law used to say it is OK to listen to, among other things, licensed broadcast radio and weather and navigation broadcasts. (found in this forum from 2003!). A search of the Ofcom site, which has taken over from the old agencies such as the GPO, shows nothing in this regard.
However, the question is "Does ATC traffic come under navigation broadcasts"? Clearly, instructions given to an aircraft by ATC must be considered "navigation" traffic. On the other hand, after 50 odd years in radio, I personally would consider it not to be a "broadcast" but a private system. After all, the users (airlines) are surely paying for these services, via airport use/navigation fees etc?
It seems to be a "law" that could keep a squadron of barristers in expensive and lucrative battles for years!