I don't know much about the Impala or how it performs safety wise. It is very cheap - but it is rear facing and meets the newer safety standard. Arguably, if the seat is rear facing and securely fitted and it meets a safety standard, then it's perfectly fine and is likely to offer good protection. I think the thing which would concern me with longevity is the position of the seatbelt when it's installed - you wouldn't want it to rest on the child's head.
Graco Extend R129 / Joie Steadi R129 are I believe the same seat. They are well made, look like they fit DC snugly (this is good) and meet the newer safety standard but some MNers have said they don't last to the advertised 105cm - more like 98-100cm, because the headrest doesn't go up very high.
Graco Slimfit R129/Joie Every Stage R129 (Again I believe a copy of each other) have a much larger harness capacity, but are limited to the 105cm standing height. They also have decent weight capacity at 21kg in the rear facing mode. However they are quite bulky front to back, I understand. You also may experience issues if the seatbelts are short.
Joie Stages is an older model, the older safety standard R44, which means that you can go over the 105cm height as it doesn't have a height limit. It is limited to 18kg weight capacity. It can sometimes be cheaper due to being older. IIRC Joie Convoy for Halfords is the same seat as Joie Stages. Again you can come up against issues if you have short seatbelts or a lack of space front to back. There is a "short seatbelt method" to install.
I wouldn't say any particularly is better than the other - it just depends what you want/need and like out of the features, price, feel of them to use etc. With seatbelt fit it can be a bit getting the right seat for the car, more so than isofix.
You'll want to look out for buckle crunch which is where the hard part of the seatbelt/buckle lies over a hard plastic or metal piece of the car seat - this is not allowed and means your installation is not safe. So try them in the actual car if you are able to. Hopefully Halfords' staff are trained in this, but I've heard some bizarre stories about people's fitting experiences there.
Axkid Minikid 2 is cheaper than Minikid 4, but the Halfords seats are around £100 or less so may be better short term options if you are on a tight budget.