The "blue plaque" mentioned is normally emplaced by National Heritage. They commemorate individuals rather than events, and the individual must have been dead for 20 years so that their significance can be judged with rather more objectivity than is likely to be gauged by their contemporaries: http://www.english-heritage.org.uk/server/show/nav.1498

Of course there's nothing to prevent an individual or organization (the local council in the pictured plaque from the BBC's erstwhile "Radiolympia" building) from putting up their own blue plaque to historic events or living persons, but it's the exception rather than the norm and it's not a "tradition" as such.