You mentioned Isofix. Any seat with isofix mountings is far superior to the conventional type which is restrained by adult seat belts.
Consult the vehicle manufacturer for best advice and recommended seats.
I have lots of concerns regarding chld seats restrained by adult seat belts. Especially with the booster seats required by older children who are under a certain [arbitrarily detemined] height and/or weight.
In the unfortunate event of a severe impact, the seat belt pre-tensioners might well fling the seat sideways from under the child and send the child diagonally forward from their seated position. An adult belt alone would not do this if correctly adjusted and fitted.
Inertia type belts are not activated according to the mass of the wearer - it seems that everyone involved in the passing of such silly UK legislation has no understanding of physics.