135cm // 12岁以上。两者满足一条即可。不用座椅。这是法律规定。Children must normally use a child car seat until they’re 12 years old or 135cm tall.连接https://www.gov.uk/child-car-seats-the-rules/using-a-child-car-seat-or-booster-seat
I suggest you to get one safety chair for your kid unless you are really confident for his/her safety without that. It is for your child, not money sake.