It's a 'standard route' diversion sign.
If a major road has to be closed for any reason, the necessary diversions have been planned in advance. So, rather than have to put up hundreds of diversion signs, at lots of junctions, the Highways Agency (or County Council, as appropriate) can simply put up signs at the point where the road is closed, stating (for example) "For Nottingham follow <diamond symbol>. For Retford follow <square symbol>".
http://www.ukmotorists.com/diversion_signs.asp
Chris