We got this free with our pram... isofix base is expensive as well (we bought on prime day)...
I went for the Joie I-anchor advance as I got a glitch deal on it and the isofix base from John Lewis in May. Been sat in the boxes since then. Was meant to be £199 but came through at £54.99 https://www.johnlewis.com/joie-i-anchor-advance-group-0-1-car-seat-black/p2663723?searchTerm=Anchor+advance
Out of interest which next stage seat are you going for?
Tbh it's to do with the straps. They just dig in to her and never look comfortable. We've changed the padding and still not great. One strap is always longer than the other too
heat added did well in the crash tests. You can get a base if you want one. Only compatible with silver cross pushchairs and unless you find one on offer they tend to be expensive.