our laws are diferant,
hospitals dont allow you to leave without a suable carrseat for a newborn, that came in not long before i had my last child who is now 16yrs

Children under three years of age must use a child restraint (see below) that is appropriate for their weight (when travelling in any vehicle). Rear-facing baby seats must not be used in a seat fitted with an air-bag (unless it has been deactivated).
In vehicles with fitted seat belts, children three years of age and over (up to 135 cms/4ft 5in) in height must use the appropriate child restraint.

• baby seats - rear-facing and for children up to 13 kgs (approx age birth to 9-12 months);
• child seats - forward facing and for children 9 kgs to 18 kgs (approx 9 months to 4 years);
• booster seats - for children 15 kgs to 25 kgs (approx 4 to 6 years);
• booster cushions - for children 6 years and up.

Seat belts are best for people over 5ft/150 cms tall, and with an adult bone structure. Children under that age or height need child seats and boosters to put them in the right position to fully benefit from the protection offered by adult seat belts.