Didn't look into UK regulations specifically but I believe they are not that much different than of any other ITU member.
FYI, it usually isn't illegal to
listen to transmissions not intended for you as such (that would be very hard to enforce - some countries/regime did and do try, though!), it is very much illegal to
disclose their content to 3rd parties (= publish it online). That's part of the telecom laws pretty much everywhere - it relates to the telecommunication secrecy, following from the requirements set out by the ITU rules which require member countries to protect the secrecy of communication.
EDIT: Apparently in the UK
it is illegal to listen to anything else than stuff meant for general reception. The above linked document is quite explicit about it.
(this one:
http://webarchive.nationalarchives.gov.uk/20150703225654/http://www.ofcom.org.uk/static/archive/ra/publication/ra_info/ra169r7.pdf )
Encryption (or lack of it) doesn't play a role - keep in mind that these rules have been established long before encryption for radio communications was practical. That something isn't encrypted/scrambled doesn't mean it is free-for-all.
If the transmission content is not meant for public consumption in some way - e.g. tv/radio broadcasting, various meteorological stations or beacons and similar, you typically have no right to disclose/publish the content of the transmission that you have intercepted unless it was meant for you. I.e. I can publish a recording of a chat I had on air with a HAM friend of mine (with their consent!) but I am not allowed to publish an intercepted communication of someone else.
So while I cannot comment on the specific UK law, UK is not really "behind" or "being boneheaded" here - at least not more than any other country. Services like liveatc.net (or rather people feeding them the data) are pretty much skirting the law in many places on this.
When talking about the airband listening, the transponder ADS-B data is usually tolerated because it is often considered as "public information" meant for collision avoidance and navigation by all users of the airspace. However stuff like ATC conversations, ACARS data and similar are a very iffy thing to publish. It then depends only on the local regulator whether they decide to throw the book at you or not.
To WaveyDipole:
Legal issues aside, if you want to listen to police, fire or ambulance traffic you will most likely be out of luck with a simple scanner. Unlike in the US, in Europe most of these services today use digital systems where you will at best hear some chirps and noise. Many are also going to be encrypted, for obvious reasons.