All those who require a definitive answer should read
this, which although being suitably confusing
does offer an email link if you wish clarification.
You may also wish to check
this link which quotes Section 5(b) of the Wireless Telegraphy Act 1949 (in PDF format).
which
"makes it an offence if a person "otherwise than under the authority of a designated person", either:
"(i) uses any wireless telegraphy apparatus with intent to obtain information as to the contents, sender or addressee of any message whether sent by means of wireless telegraphy or not, of which neither the person using the apparatus nor a person on whose behalf he is acting is an intended recipient"
This means that it is illegal to listen to anything other than general reception transmissions unless you are either a licensed user of the frequencies in question or have been specifically authorised to do so by a designated person. A designated person means:
a. the Secretary of State;
b. the Commissioners of Customs and Excise; or
c. any other person designated for the purpose by regulations made by the Secretary of State.
or:..."
We have another thread about this
here courtesy of the ATC forum.