I've seen this post on Money Saving Expert relating to free Mr Kipling Cakes, it mentions Manchester Piccadilly and Tottenham Court Road. There's also other locations in London, Birmingham, Sheffield, Nottingham and Glasgow - some of which may be near transport hubs.
http://forums.moneysavingexpert.com/showthread.php?t=3867671
http://forums.moneysavingexpert.com/showthread.php?t=3867671