"It also takes your photo on entry. Quite why I'm not sure"
My understanding is that this is purely a "Gate-side" security measure. Everyone entering "Gate-side" has their picture taken and it is associated with their passport/boarding-card/whatever. Then, as they leave "Gate-side" their image is taken again and the two compared (usually manually). It makes sure the person who enters is the same as the person leaving.
Someone tried to explain this to me once. It was to do with preventing the following scenario.
Person A is a British citizen but is prevented from travelling to, say, Germany (maybe a football hooligan).
Person B is also a British citizen and can travel where they please.
Person A books a flight from Manchester to London.
Person B books a flight from Manchester to Berlin.
Both check-in and pass through security. They then meet up gate-side, swap boarding cards and person A can now travel to Berlin.
Except, with the new system, when person A gets to their gate, the staff scan the boarding card, pull up the picture taken when they came through security and the two pictures don't match.