Recording audio on CCTV is generally not legal in the UK - that said mine record but with the sensitivity turned down so they wouldn't pick up a conversation on the street.
I don't believe it's illegal,that would involve criminal sanctions. The ICO has issued
guidelines saying that it should not be recorded. Whether there are other laws that actually make it illegal I wouldn't know. I could foresee it being actionable as a civil matter by a lawsuit from an affected individual for breach of EU privacy laws. Whether the ICO can punish businesses for breach of it's guidelines, I don't know.
However, as I also understand it, the ICO regulates Data Protection. I understand that Data Protection Law covers business users only, only not private individuals.
I would have thought there was still a risk with an individual of getting sued for beach of privacy. I wouldn't know if it's legal (from a civil standpoint) to record audio restricted to pick up from your premises only. I have heard with CCTV for a private individual the camera should be positioned so it doesn't point into the street, although if the street is captured incidentally to the main area of focus, then that is ok.
I guess you'd need a specialist lawyer for a definitive answer on all of the audio scenarios / law.