3-door cars are tricky for spin seats. The best one usually suggested is the Britax Dualfix, but that's quite a bit higher than your budget - about £225+ depending on which model you get.
The problem with the Joie one is that the handle to release the spin is where the child's feet are, so you'd need to get totally into the back to do it anyway, and then you have to spin it so that they face where the door would be in a 5-door car, if you try to spin it so that they face the middle back seat, it gets stuck on the side of the car - I know because I tried this in our car while we were in a car wash and DC got scared! We have a 5-door but very small car.
The handle to release the spin on the Britax is on the base itself, so you can reach it wherever the seat is pointing (but you still have to spin it towards the car window, and a toddler's legs will stick out).
The handle is on the base for both Cosatto ones (they have two different rotating seats), but the one that's £120 doesn't rotate. Only the more expensive version over £200 rotates. And I'm not keen to recommend them for toddler seats, because their seats that have been crash tested by ADAC didn't do very well unfortunately. So I would stick with the Joie 360 or the Britax if you want to look at spinning seats.
So I think in a 3-door car usually you'll find you have to sit in the back with the child in order to get them into the seat, if you can't find a seat with a spin function that works (and I would highly recommend trying this out for practicality and front passenger space before you buy, so possibly waiting for now.) So another option to consider might be one of the larger extended rear facing (ERF) seats which go up to about 5-7 years (25kg seats), rather than the seats you're looking at now which go up to 3-4 (18kg/105cm seats). These start at about £160 for the Britax Two Way Elite or (sometimes) Britax Max Way on offer (usually £180), Axkid Move is also worth looking at which is about £200 but sometimes £180 on offer. The larger seats sit a bit more upright in the car, so they give you more space in the front seat but they can also be situated a bit more away from the back of the car, and tend to have low sides, so they are easier if you're trying to manouvre a toddler in from sitting in the back next to the seat! Isofix rear facing seats are quite high up in the car. I'd also think about what you'd do if you potentially have a second child - how would you manage getting the older one in and out with another child seat in the way? (Of course changing car might be another option in that situation).
Something else to think about - if you think you're likely to get totally fed up with the rear facing seat and turn it forwards (if indeed you buy one which can do that) I would absolutely check the ADAC safety report on the seats - any rear facing seat is a good bet, but once you start forward facing it's well worth comparing safety ratings as they start to make a big difference to the performance of the seat then. If you are 100% committed to keeping rear facing it doesn't matter which rear facing seat you get, the only safety difference is in side impacts then. I would also recommend you don't go forward facing until a bare minimum of 15 months in line with newer regulations, preferably 18 months/2 years, but the longer the better really.
It's not true that all isofix seats fit in any car with isofix. You also have to take into account the angle of the seats and positioning of isofix points. If your car is newer than 2013, it might have some "i-size approved" positions, if it does, then you can definitely use any seat approved as i-size in that position. You can tell by a little label next to the isofix point which looks like a child car seat and a letter "i". If your car isn't i-size, then you'll need to consult fitting lists in the absence of the option of a trained fitting. Most car seat manufacturers publish their fitting lists on their website, although they might not be up to date or complete. You can also email the car manufacturer to see if they have any preferred seats to recommend, and the car seat manufacturer to ask if they know whether the car seat is approved for your car. The other issue is that although fitting lists/i-size lists can tell you whether the seat will be safe to use, it doesn't mean it will definitely be practical or comfortable. You might find the recline is non-optimal or the front passenger ends up with their knees against the dashboard! So particularly when you are pushed for space it can be well worth having a fitting in-person if you can spare the time to wait.
Another option would be to contact an independent car seat fitter/Extended rear facing (ERF) specialist, perhaps one on this list: www.carseatadvice-uk.com/specialist-retailers.html - and buy a consultation off them which you'll use when lockdown is over. That helps support a small business in a time they might struggle to trade, and you'll get the best advice on the best seat for your car. You'd pay the cost of the seat later, when you choose one. They also stock the 25kg seats.