I’ve been a victim of Bristol bike theft.
I’ve wondered about a registration scheme with the local police service – photo, frame number, brief description. So that when or if bike is stolen there is a collection of evidence linking you with the bike and something for the police to work on. With a description and a frame number it might be easy for the police to stop and examine a suspect bike and exclude as necessary.
Also, as above. Get ebay to insist that frame numbers are explicitly recorded in any advert for bike or frame sale.
I know there have been initiatives where police go to peoples houses to offer advice about burglary prevention – perhaps something similar wrt garages/shed security. I know i have put in a considerable amount of new security – only time will tell if it is effective