No test case required. The UK Wireless Telegraphy Act, 2006, Section 116 states:
(1)In this Act “wireless telegraphy” means the emitting or receiving, over paths that are not provided by any material substance constructed or arranged for the purpose, of energy to which subsection (2) applies.
(2)This subsection applies to electromagnetic energy of a frequency not exceeding 3,000 gigahertz that—
(a)serves for conveying messages, sound or visual images (whether or not the messages, sound or images are actually received by anyone), or for operating or controlling machinery or apparatus; or
(b)is used in connection with determining position, bearing or distance, or for gaining information as to the presence, absence, position or motion of an object or of a class of objects.
Receiving ADSB-out and Mode-S signals, unless you are an intended recipient (as a Flying Club clubhouse would be) is definitely covered by the Act and hence illegal in the UK. Previous versions of the Act only
using the information gained was an illegal activity, but the 2006 Act made receiving the information illegal as well.