Sadly, no mileage really.
To be effective, the watermark/metadata needs to be applied by the publisher (and any subsequent publisher) and not the creator. Most professional creators (i.e. those making IP that has actual value) are already smart enough to mark their property.
Think about what a picture goes through, commercially. It might get cropped, retouched, colour-corrected, flipped, scaled, collaged; it could be re-saved into a format that doesn't support EXIF or similar; or it could be embedded in a container such as PDF or SWF. But legally, it's still the same picture.
Any subtle watermark or metadata applied by the creator runs the risk of being deleted as part of a commercial production process; while any mark that is unsubtle is liable to render the work unsaleable in the first place.
One alternative to the database that's being proposed puts the onus on the publisher rather than the creator and goes like this:
All publishers need to be forced to publish a rights credit along with any picture they publish. And ideally that means a human-readable credit embedded on the picture; plus machine-readable metadata as well. And the definition of publisher needs to be extended to cover everyone (much as libel does), and not just the big players.
Some photo libraries such as istockphoto (owned by Getty Images) already have clauses in their Ts&Cs that penalise customers who pay for picture licences but publish the pictures without source/rights attribution.
It's do-able in law but not likely to happen because the Tories will bleat about "Red Tape" harming business until your ears bleed.
So instead the creatives get tied up in blue tape. Thanks, Dave.