there are exceptions which allow for the 'emergency' transport of a child as you mention.
1) in a licensed taxi or private hire vehicle;
2) if the child is travelling on a short distance for reason of unexpected necessity;
3) if there are two occupied child restraints in the rear which prevent the fitment of a third.
In addition, a child 3 and over may travel unrestrained in the rear seat of a vehicle if seat belts are not available.
read up about it at
http://www.childcarseats.org.uk/law/fromseptem ber06.htm#over3upto12