Those cheaper ones that claim to do everything (spin, have isofix, last for all stages) are generally poor quality and are to be avoided. Unfortunately they seem to be flooding the market at the moment, I think because car seat laws have recently come in in some Asian countries. They are being heavily marketed here too because we use the same regulations, on places like Bounty and Facebook. Bit of a nightmare if you're in car seat safety because how can you convince someone it's worth spending 2-3 times the price on something that appears to be the same product? But they often have compromises both in safety and in practicality - you just get a better product if you stick to established brands with a safety reputation.
You're limited to either 18kg or 105cm with spinning seats on the rear facing/harness function, it's one or the other, even if they have a higher weight limit they'll have the height limit and most children reach 105cm slightly before reaching 18kg, so you don't gain anything from it. The ones which last longer just turn into (often a poor performing) high backed booster seat after this. You can get a booster seat for £30 later and it won't weigh 15kg or be a nightmare to store or install.
Joie i-Spin for £220 is a fantastic deal! I've never seen it under £280.
Other spin/isofix seats up to £250 I'd recommend:
Joie 360 Spin
Joie Spin Safe (same seat but RWF only, has better side protection)
Britax Dualfix 2R
Cybex Sirona S (I don't like it, but it's perfectly safe and many do)
Another angle to consider would be a rear facing seat up to 25kg instead. These last approx 2-3 years longer. You generally need a child to be in 12-18 month clothing before they'll fit into one, so your 7 month old might be a little small, but she might be OK especially if you can hold off for a couple more months. They do not spin, but the sides are very low and they sit more towards the front of the car so you have much more space to get the child in. The installation can be more complicated.
Up to £250 in this category:
Britax Two Way Elite
Britax Multi Tech 3
Britax Max Way
Axkid Move