Car seats for older kiddies? (Manama)

Hi there,
I'm looking for a highback booster seat for my kid (aged 4). So far in Mothercare, I've only found the cushion booster seats. Back in the UK, from December, the cushion boosters are going to be made illegal for younger kids because they are not as safe as the highback version. Being paranoid about car seats I'm reluctant to use them (but they are definitely better than nothing).
Can you recommend where else I could try looking?
Thanks so much,
Laura