First of all, even in the UK, the burden of proof lies with them, since they are illegally withholding your images.
But I suppose you can convince them by showing that you have the files. If you use the auto-comment feature of your camera to inject something to identify you in the EXIF file, you have established your case.
Furthermore, the EXIF data on your files should show the camera you used, further proving your point - if you choose to indulge in their paranoia and be polite about it.
I would suggest though, that if you choose to prove they are yours, call the police first and prove it to them instead.