Try the Facebook group car seat safety UK. It's really helpful and I've learnt lots I didn't know about car seats and safety standards / impacts in a crash. At 3.5 years they should ideally be rear facing for a bit longer. The brands normally recommended there are the axkids, avionaut sky, or besafe stretch. Depends on your child's centiles and your car though, as to which is best.
We have the axkid movekid … lasts up to 125cm/36 kg and the axkid one3 … lasts up to 125cm/23kg.(whichever is reached first) Based on my daughter being on the 25 centile the 36kg seat will last till she is maybe 7/8 and the 23kg seat till she’s possibly 6/7. I plan on keeping her rear facing as long as possible until she’s possibly has outgrown these seats
There are a few more out since that last pic but gives you a good idea. There are often good deals to be had from the independent car seat specialists….
Following along for recommendations for when Elodie grows out this same seat 🤣 Ideally ones that won’t break the bank!