UK: Can someone record or broadcast over the phone a private conversation from outside of the room where this conversation is take a place?

Depends on who they are

There are virtually no limits on individuals filming or recording anything they like providing they do not commit an offence (like trespassing) to do so and they are not invading your privacy. If someone is talking loudly enough that they can be heard through the walls then this is not a private conversation. That is, if the recording device is in your private space yet it records conversation coming from their space (assuming that this is just ‘overheard’ rather than being deliberately sought out) then it isn’t private.

Businesses are regulated in what they can do and they must get consent to record you over the telephone (but not in person) unless the recording is for a specified reason.

However, while it's generally legal to make the recording, sharing it may be illegal.