Children may travel in the front passenger seat subject to the rule that a child who is both under 12 years old and less than 135cm in height must use the appropriate type of child restraint.
Child restraints are divided into 4 categories. These are rear-facing baby seats (which are officially known as Group 0 or Group 0+ child restraints), child seats (known as Group I child restraints), booster seats (Group II) and booster cushions (Group III).
The definition of 'appropriate' is dependent upon the child's weight and not his/her age or height. The full details are here:
http://www.thinkroadsafety.gov.uk/campaigns/ch ildcarseats/childcarseats.htm
(See section 3 to determine the appropriate type of restraint for your child).
Chris