This says its the arresting officers job to secure them:
http://www.dailymail.co.uk/news/art...ng-protocol-fears-wife-children-attacked.html
That assertion came from a "family source" of the van driver. It might be true, but it makes more sense to me for it to be the responsibility of the two police van officers to secure arrestees in the van than for bike officers who need to get back to work asap.