UK law on photographing things

There's a photo of me looking ecstatic that I'd found a ten pound note (most likely mine anyway), and I was displaying the tenner in question to the camera, is that illegal?

actually, you can photo banknotes. however, the resultant image must be 50% larger or 50% smaller than the actual banknote.

EDIT: apparently this is actually an urban myth and you have to obtain permission to photo banknotes of any scale

The register never actually says that's illegal but...

The film in an analog photochemical camera reproduces an image of the banknote, as does making prints by any process. I believe the problem is determining when an image containing a banknote becomes a forgery attempt, at which point the +/- 50% guideline may be one way to successfully argue the intent was not forgery.