Have you checked up with the latest changes in UK law regarding Car Seats?
Currently booster cushions fall under the Group 2/3 car seat category, meaning they can be used from around 4-years-old. The new extension recommends children use backless boosters once they are 125cm or taller, which is roughly about 7-years-old.
Also, all children below 15 months old must now travel in a backwards-facing car seat.
All children must use the correct child car seat until they're 12 years old or 135 centimetres tall, whichever comes first. Children over 12 years old or more than 135cm tall must wear a seatbelt, and should not travel in the front of the car unless no other seat is available.
You must make sure that any children in the vehicle you’re driving are;
In the correct car seat for their height or weight until they reach 135 centimetres tall or their 12th birthday, whichever is first.
Wearing a seat belt if they’re 12 or 13 years old, or younger and over 135cm tall.
These articles have further information;
Please check the government website for up to date details;