I don't think the consent of both parties is the issue at all actually.
That would make filming anything in public illegal if that interpretation held up. The only legal question is whether that is a "private conversation".
Considering the implications on democracy as a whole if it was found to be a "private conversation" I would gladly bet that it wouldn't be considered one.
Filming is fine. It's only the audio, that's questionable in this circumstance, which is why I posted above about disabling it.