Join car seat safety uk group on Facebook and they will give you recommendations for a new car seat based on your babies age and size as well as budget. You can also post a picture of your current car seat fit and they can make sure it’s a good fit.
What car seat do you have? Do you have a picture?
What ever you choose, try to rear face as long as possible. It's the safest way for baby to travel. I will be rear facing my LO till 7 years old.
I’d say, go to a specialist retailer where they’ll let you try out car seats. So much better if you can put baby in, they should help you get a good fit too. They should also help you try the seat out in your car.