Article 21(4) of the Air Navigation Order. See also Article 26(2) and note that it does not apply to FAA licences.
http://www.caa.co.uk/docs/33/CAP393.PDF
You shouldn't believe everything that you hear on the phone (or everything you may read on websites, for that matter), but the ANO is a piece of paper invested with a special magic by virtue of Brenda and her little helpers.