Doona
A cheap one is the cosco brand. Idk if you have it over there. But I hear a lot of people talking about how it’s great but super lightweight and easy for travel.
I haven't done research into it myself yet but I have seen the Maxi Cosy Nomad Plus, it's a fold up one
The Joie Steadi is quite a good one for swapping between cars. It won’t last long if your child is high centiles but our high centile son got to about 2 and a half in it, I think it’s an 18kg limit. it’s a safe brand, RF, cheap and belts in really easily
I'd recommend joining the fb group "car seat safety uk" for advice and recommendations, they'll advise seats based on your LOs height/weight, budget etc x
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