Although these are not stamps as such, you've got to admire Royal Mail's inegnuity for tapping into 2012's Gold Medal mania and allowing Smilers customers the opportunity to create something different.
(UPDATE 8 Feb: now or shortly also available on Android platforms. And despite the lack of any publicity from Royal Mail to my knowledge, these are selling well!)
Our reader supplied us with images from the App (click to see larger):
1. select type of label
2. Note only the Union Flag stamp is available with this label
3. Add caption
4. Order multiples of 10 or 20 stamps
UPDATE 6 February: Thanks to a reader we can report that