I don't know a huge amount about it, but I have gone down a rabbithole about this before - as I understood it, it's mainly infant carriers which tend to have flame retardants in, because under UK law they come under the furniture & furnishings law as they can be used on a pram and a pram is classed as furniture because a baby might sleep in it. (The whole law is rather old fashioned).
I don't think that seats which stay in the car tend to have this, but if worried, the seats shipped from carseat.se might be a better bet, because they will be the EU versions rather than the UK market versions. The car seat regulation is the same, which means they can be used in the UK legally, but the fabrics are sometimes different, particularly on infant carriers.
The other thing you could look at is whether the seat has been tested by ADAC which is a German organisation who sell their results to Which? etc - you can look them up directly on the ADAC website. Their test for what they consider harmful chemicals is "Schadstoffe" and it's a pretty strict test. I don't know what exactly is tested for but PFAS are mentioned on one of the articles. This is their own description of the test (google translated)
All parts with which the child comes into contact (cover) are tested for harmful substances . Based on the GS test, the "REACH" directive, and Ökotex 100, the content of PAHs, phthalates, flame retardants, phenolic compounds, organotin compounds, AZO dyes, and heavy metals is tested and evaluated.
In addition, since 2025, the seat upholstery fabrics have also been tested for their content of environmental pollutants (PFAS) according to the hydrolysis method specified in the draft standard DIN EN 17681-1:2023-12.
Axkid One gets a perfect score on this. It's an older version than the current one, though, and was before the 2025 change.
Besafe Beyond does as well, and was tested in 2025 so the most up to date version of the test.
There have been 4 belt-fitted ERF seats tested by ADAC which are still on sale. (Axkid Minikid 3 and 4 are basically the same seat, I can't remember what changed, not the covers though.)
Besafe Stretch - perfect
Axkid Minikid 3 - "good" (not perfect score but not too many)
Britax Safe Way M - perfect
Avionaut Sky 2.0 - this didn't do v well on the Schadstoffe test.
All of those were before 2025.
Joie don't have an ADAC test on the i-Prodigi or the i-Soren, but their other seats all to get good or perfect results on the Schadstoffe portion of the test, though I also have a feeling that they use different versions in the UK and I got the impression from someone I worked with in a baby store that this was to do with flame retardants used in the UK versions, but there is also a high chance that I completely misunderstood this, or it only referred to the baby seats or something like that. It was also 5 years ago so could be outdated now.
One Graco seat failed the test purely on Schadstoffe in 2023, that was the Snuglite which is an infant carrier seat. (Graco and Joie are the same manufacturer).
A Besafe seat up to 105cm also failed this in 2023.
Because this is an issue which is getting more publicity you might find info on the manufacturers' websites, too. And I have heard Axkid are very responsive if you email them, I don't know about the others. It's certainly a popular topic in Germany, so possibly the German language versions of the websites might have info if the English language versions don't.