We have got the Cybex Aton 5 for our 16 week old. It doesn't seem that easy to find in the UK but it's been tested by Which? apparently, so it seems like it is available.
It gets great safety results, including a fancy little side impact flippy-out thing that I don't really understand but apparently makes it safer. That was my first priority. Other than that -
I really like:
The headrest adjusts the harness, so you don't have to rethread it ever as it just moves up and down.
The headrest itself is easy to adjust and you can even do it with the child in the seat which is handy to get the right fit. Lots of little increments rather than big ones, which means the harness always fits snugly.
It seems really comfortable, DS2 hardly ever complains in it which is really unusual for a baby IME! He even looked fine in it as a newborn, not drowned, and at 7kg he still has loads of space left in it so I'm not worried he'll outgrow it early.
Fits on Maxi-Cosi adapters so it's compatible with a wide range of pushchairs.
Base can be isofix or belt fitted which is handy - you don't have to buy a separate one.
I really like the look of it too. DH said it was space agey, and shoot me, I like the colour!
Niggles:
It's heavy. Much heavier than the basic Mothercare seat I had 10 years ago with DS1. However I assume this is down to it being more robust, and we often use the pram wheels to transport it from car to house anyway.
Straps can be a little stiff to tighten. I often have to push the crotch strap inwards to create slack in the shoulder straps so I can tighten it up from there. If you weren't aware of the need for seat straps to be totally flush I could imagine this might be missed, which is a concern for safety.
The headrest kind of sticks out from the seat which means it pushes a newborn's head forwards. This isn't an issue once they are a bit bigger, though, and I never noticed any trouble breathing when DS2 was tiny.
The newborn insert is just a piece of polystyrene when it's removed. It does the job well but I'm a bit concerned that I am likely to lose or break it or accidentally throw it away while in storage for potential DC3, because it isn't obvious that it's a part of the car seat and there's nowhere built in to store it when it's not in use.
I never find it clicks into the base very easily - but might be just me as DH manages just fine.