I have some legal documentation from my leasecar company:
"Know the New Child Seat Laws
From 18 September 2006 the law regarding the use of child seats in cars changed in England, Wales and Scotland (and will change in Northern Ireland later this year). Child seats or booster cushions are now required for all under-12s less than 135cm (4ft 5in) tall.
Children under 3 years old
In the Front Seat
The child must use the correct child restraint (baby seat or child seat). It is illegal to carry a child in a rear-facing baby seat in a front seat which is protected by an active frontal airbag.
In the Rear Seat
The child must use the correct child restraint (baby seat or child seat).
Children aged 3 years and above, until they reach either their 12th birthday or 135cm in height
In the Front Seat
The child must use the correct child restraint (booster seat or booster cushion).
In the Rear Seat
The child must use the correct restraint (booster seat or booster cushion), where seat belts are fitted.
There are three exceptions where there is not a child seat available. In each case the child must use the adult belt instead. They are:
- in a licensed taxi or private hire vehicle
- if the child is travelling a short distance for reason of unexpected necessity
- if there are two occupied child restraints in the rear which prevent the fitment of a third
In addition, a child aged 3 and over may travel unrestrained in the rear seat of a vehicle if seat belts are not available."
Looks like you could work around this legally, but as NAB3 said the child is probaby safer in the back in terms of the height of the airbag etc.