ds went into one just before 4 - Maxi Cosi Rodi -and still uses it with the back support, you can buy a cup holder as an extra !
dd is almost 4 but doesn't meet the minimum weight for most yet (15kg +) and the ones from 9 months don't come out well safety wise although might consider Supercruiser.
Halfords and Boots have some reduced atm.
they need to weigh 15 kilos and if the seat uses a normal seat belt - these are the ones you are talking about aren't they? they need to be tall enough so it goes between their ear and shoulder. I did buy a booster and seat belt adapter thingy which moves the seatbelt across for my DS but he is 4.5 and very tall for occasional emergency use.
Otherwise I would stick to one with a back and headrest for normal use