Primary position

The daily Telegraph puts it so well:

One of the purposes of primary position is to prevent motorists from attempting to overtake. This is often essential for the safety of both the cyclist and the motorist. The minor inconvenience suffered by a motorist ‘held up’ by a cyclist in primary position is more than compensated for by the safety of both parties.

http://www.telegraph.co.uk/men/active/recreational-cycling/10604525/Cycling-advert-ban-why-cyclists-must-take-the-middle-of-the-lane.html