Yes, indeed, not only have I heard of this but have had dealings. Firstly, and this is not a criticism of you because you probably didn't know what to do for the best, but this is the best way forward in case you're unfortunate enough to experience this again:
Incidentally, I obviously don't know which Police you're dealing with, but from what you say above, their attitude stinks, because I know for a fact that courts do indeed frown upon what's known as "theft from employer" for which there's a specific offence on the statute books within The Theft Act.
1: If at all suspicious about an employee, monitor your cctv as much as possible, make notes of times, dates etc when suspect's on duty of any untoward behaviour which suggests impropriety with any of your stock as well as cash.
2: If so, get your partner as well to view the video in question for evidential purposes, and keep the video to one side.
3: Make sure that you cash up at the end of every working day and that you're aware of how much is in the till when the suspect begins work. If you have till rolls, hold onto the relevant ones in case the suspect tries to claim that all the transactions were valid and above board. Your till rolls can help to disprove that.
4: Without bringing it to the suspect's attention, the best thing to do would be to arrange to go in person to your nearest Police Station, taking any evidence, like the video(s), till rolls etc with you, and request the Police to start the ball rolling by taking a written statement of complaint from you and offering them the evidence you've brought.
5: The Police ought then to act on this complaint by arranging to have the suspect arrested and interviewed. The tricky bit will be whether or not you would be comfortable with it being done at work, or letting them have his/her home address. At the same time you would be perfectly within your rights to serve the suspect with