My apologies for my snappy comment , sadly text on the internment can give the wrong impression ,I know you didn't, I did, it was a tongue in cheek about people in the US seamingly getting away with serious crimes on such technicalities, bit vague for the internet, my fault.
I feel for your friend, not displaying cctv signs shouldn't be a reason for the evidence not to be used in a criminal case no matter whether it is for business or personal use, public or private land. Individual judges can choose to exclude evidencve for certain reasons, this should not be one of them, maybe some convoluted "lesson teaching" If you had signs it would be a deterrent and we wouldn't be in court kind of thing, although that's just my imagination.
Ofcouse admitting having recordings that breached whatever privacy/GDPR rules/laws should be a seperate matter.
I’ve a two sided view re the signage , if you don’t put it up , it may not be usable , but if you do put them up , then they may wear face coverings , which would hinder possibly knowing who it was .
I get the gdpr thing about if your filming public spaces / roads , properties , neighbours homes . But as far as I would be concerned , if you enter someone else’s property , then you forgo any of that .
Just seems a very strange law that domestic and private properties have two set of different rules . Ive the view of if your not doing anything wrong , why would you be bothered about being filmed .