Pedantic I know, but you can log it. It won't however count towards the experience requirements for the license. You have to be at least 14 for it to count towards experience requirements, at least 16 to be Pilot in Command, and at least 17 for license issue.
The ANO says you must put certain things in your logbook, it doesn't say you can't put anything else in. The licensing requirements will tell you what will actually count towards license issue.
Mike
(paid up member of the Southsea Pedants' Siciety)