Hi OP
The problem with isofix car seats for this stage is that it adds a lot of weight to the seat, so none of them are especially easy to take in and out I'm afraid.
Do you need to take it out in order to have the seat be usable for someone else? Or just move the seat between two cars?
You could try looking at seats where the seat unit and base are separate, which tends to make the install nice and simple, but they can often be more expensive than buying a seat with the isofix fitting already built-in.
Seats with separate base/seat e.g:
Joie i-anchor
Axkid Modukid
Cybex Sirona M2
Maxi Cosi Pearl (plus various versions - Pearl Pro, Pearl Smart etc)
I don't know if you have a preference on rear or forward facing? I'd strongly recommend rear facing at this age as it's much much safer for your LO if you were to get into an accident. There is one more lightweight isofix rear facing seat for this age group:
Concord Reverso Plus
But it's not that popular in the UK and you might struggle to find a supplier.
So the next type of seat I would recommend to look at would be spin type seats, as they can be forward facing for easier install/storage and rear facing for better safety in the car. e.g:
Britax Dualfix
Joie 360 Spin
Maxi Cosi Mica
Do you know how much your LO weighs? That can also be important in this decision. And will it be their main car seat, or a spare? Do you have a budget?