From the Wireless Telegraphy Act 2006, 46.1.a
[It is an offence if a person uses] wireless telegraphy apparatus with intent to obtain information as to the contents, sender or addressee of a message (whether sent by means of wireless telegraphy or not) of which neither he nor a person on whose behalf he is acting is an intended recipient
That would certainly seem to apply to ADS-B.
Incidentally, the "whether sent by means of wireless telegraphy or not" bit is interesting: it seems to extend this clause to any kind of eavesdropping.