I got the joie steadi R129 - can go in easily with a seatbelt and not crazy expensive - did see you said isofix but they’re often a couple of hundred pounds more - we have this one in granny’s car and it’s very sturdy and good!
I would recommend joining the car seat safety groups on Facebook for expert advice Bump to Booster UK or Extended Rear Facing UK Are 2 excellent groups You ideally want something Swedish plus tested
https://www.thenurserystore.com/p/puggle-lockton-360-rotate-luxe-group-0123-car-seat-storm-black-0-12-years this one’s isofix and you can use from birth to 12 years x