Car seats for babies and children are compulsory in the UK for all children up to 12 years old or 135cm tall. For small babies up to 13kg a rear facing car seat should be used. If you are fitting a rear facing car seat, you should always disable the passenger airbag. Front facing rear seat car seats should be used for children up to 25kg and then booster seats or booster cushions for larger children. All car seats must be EU approved, which is shown by a capital E in a circle.
Many car seats for very young babies come with a carry handle and double as a baby carrier, so you do not have to take the baby out of the seat to move them in and out of the car.