From March 2017 the law on buying booster cushions will change. Booster cushions offer no side impact protection and can even slip from underneath the child in a collision; it is strongly recommended you always use a high backed booster seat.
Under current UK law, all children travelling in a car must use the correct car seat until 12 years old or 135cm tall. In some European countries this height limit is 150cm. Under the new booster seat rules,, backless booster seats will only be approved for use for children taller than 125cm and weighing more than 22kg. At the moment, children weighing as little as 15kg, that’s around three years old, can travel in backless booster seats. But many child car seat experts agree that this type of booster seat is unsuitable for such young children. A small child isn’t held as securely in the seat, the adult seat belt isn’t guided across their little body in the best way, and, most importantly, a booster seat offers no protection for a child if your car’s involved in a side-impact crash.
This will be for NEW purchases , so if you have a backless booster and are using it, you can continue to use it for your child, however if you are intending to buy a booster after March and the child is under 125 cm and lighter than 22kg they will have to use a booster with a back, NOT a backless one.