No other details are available at present, with the Royal Mail informing us:
Unfortunately it is too soon to have or to send out any information regarding an issue in January 2013. Probably around end of November or start of December we should have a FIRST publication that we can send.
It won't be the first time that Doctor Who has been officially recognised in such a prestigious way. A Dalek was chosen by Royal Mail to represent television in The Entertainers' Tale issue that was released on 1st June 1999. The set also comprised Bobby Moore (sport), Charlie Chaplin (cinema), and Freddie Mercury (popular music).
(with thanks to: Richard Thomas)