There is nothing magic about Joie/awful about cosatto - Joie do some good features for cheap prices, that's all - sounds like she's been on Facebook, some of the groups are a bit fanatical about brands in a way that starts to get a bit nonsensical.
Though I agree the cosatto probably isn't the best, there is nothing terribly wrong with them either. But for £300, you have better options. In general, those seats that do every stage are a false economy and you're better off just getting something covering the current stage and then changing to a proper booster later on. (Not to mention, she might like jumping rainbow unicorns at 2, but is much less likely to want to use that seat once she is a much cooler 7, 8, + year old).
As you have a small car, I would not recommend the Every Stage as it's huge, especially when rear facing. And it's got a poor rating if you use it forward facing, so it's only really good as a rear facing seat and it's quite expensive for the basic rear facing seat that it is. If you want a basic rear facing seat, you can get one much cheaper.
Spin seats in 3 door cars sound like a good idea, but I'd try them out first to see how practical it really is. One issue can be the placement of the button which releases the spin capability. Joie 360 Spin for example is very popular due to its price, but the button to release the spin is between the child's legs - this is no use at all if you want to easily be able to reach it from the front, you need one with a button on the base of the seat or on the side.
Off the top of my head under £300 that's
Joie i-Spin 360
Britax Dualfix (all versions)
Maxi Cosi Mica
The second thing you want to check is whether it can even spin with the child facing into the car - they are designed to spin with the child facing the "door". We used to have a Suzuki Swift, 5 door, but one day we were in a car wash and tried to turn DS2 to see us and the back of the seat protrudes out so much that it would not physically turn past the window. Once they are past 8/9 months their feet stick out the edge of the seat, so you wouldn't want to be turning it that way as it would bash her feet into the wall. So definitely do a fit check before you buy a spin seat. It seems like a perfect solution, but might not actually be that practical. Lastly, check if you can even swivel it to the forward facing position with the headrest in position for a younger child - some have a locking mechanism tied to this, to prevent you forward facing too early.
If you decided to get a non spin seat, to use a rear facing seat you would usually put the front seat forwards, hold the child on your lap and sit in the back middle seat to place them in their seat, then climb out again. Or sometimes you can put them in through the boot (!) but that seems like it would be annoying.
In terms of other rear facing options, you can get a basic rear facing seat from Joie/Graco (Joie's parent company bought Graco a few years ago and use their branding, they aren't the same as the old Graco seats) for about £100. The main complaint about these is that the seatbelt path for them means the seatbelt is in the way of the door - but - in a 3 door car, you would get them in and out from the opposite side. I have actually done this as our travel seat fits like this, and it's fine. These are lighter and easier to transfer between cars than the spin seats, but they are quite reclined and don't have a lot of leg room for the child.
Or the "gold standard" for rear facing seats is probably a Swedish type seat - these are unusual so you probably won't have seen them, but they are also fitted with seatbelt, along with tethering to the front seat. To move these easily between cars, you simply buy a second set of tethers and install those in the second car. Once the tethers are fitted, you just plug them into the seat like a seatbelt.
You've got 3 options under £300:
Britax Max Way
Axkid Minikid 2
Axkid Move
These can be more comfortable because they fit compactly and they sit more upright and are designed with older children in mind so have much more leg space as well as growing space up to approx 125cm (this is a good couple of years longer than the standard, 105cm/equivalent to old Group 1 seats - if your son is tall, you may have found him outgrowing younger than you would have liked.) The "downsides" (some may disagree!) are they are only rear facing - no option to change to forward facing, until you buy the next seat, and you wouldn't be able to use them yet, because these are like the old Group 1 type seats in that the child needs to be at least 9kg, sitting unaided and at least about 80cm tall, because they only have a very minimal recline, not a full recline like a baby seat. So you'd need to stick to the baby seat until she outgrows it. There are seats in this category that can be used from younger, but they cost more than £300.
You might want to consider 2 different seats as moving toddler seats can be a bit of a pain.