If you have £400 budget in total, then Britax Dualfix M is a big chunk of the budget as it's around £300. I was going to say if you have £300, it's worth looking at the Dualfix M Plus which is about £325, or the Besafe izi Turn which is on offer at the In Car Safety Centre for £309. - The Besafe doesn't need to touch the seat fully at all plus it's just a really lovely spin seat, and the Dualfix Plus has a higher weight limit of 20kg.
But for £400 total I'd probably look at cheaper models. Cosatto are generic seats, so they tend not to do as well in testing and the Come and Go has to be in the reclined position when rear facing which is a bit of a pain. It was ADAC tested under the name Heyner Multifix Twist (generic models are sold under different names in different countries) and it didn't get a very good score, even rear facing was mediocre, probably similar to a purely belt fitted rear facing seat. Let me clarify - when I say a mediocre rear facing score, this is still better than even the best forward facing score. So if you only have a small budget for rear facing it's still worth doing.
Graco Turn2Me i-size is probably the best budget model on the market at the moment, it's £160 from their own website and has the R129 side impact protection. Leg room is a little lacking but other than this a really nice option. Seems to be a clone of Joie 360 Spin with added side protection.
Maxi Cosi Mica Eco is £230 at Amazon at the moment, which is a great price. Graco Turn2Me i-size is also worth a look at £160 - that has the improved side impact compared to the original Turn2Me or Joie 360 Spin.
If you want a spin seat in a 3-door, you will want to check the position of the button or release for the spin. Cybex and GB spin seats are out, because you have to push up between the child's legs. Joie Spin 360 and Graco Turn2Me (including the i-size) also have the button below the child's feet, however Joie i-Spin 360 has a button on the side. Britax, Besafe, Recaro, Maxi Cosi all have the button on the car seat base so it's reachable from the front.
The other issue in a 3-door is checking whether the seat will actually spin. We had a Suzuki Swift, 5 door, but I tried to spin DS2 around in a car wash once and the seat kind of protrudes at the back, so it did not physically fit past the door side. So a fit check might be essential. I don't know how the more budget models fare here, but you'd definitely want to avoid versions with top tether. Sometimes it can make more sense to just get a static seat and take the child into the back on your lap, then sit them in the seat from the side. The Joie belt fitted models work well, as the belt is on the other side, or 25kg+ ERF seats as they have lower sides as a rule.
Joie Tilt has quite a short shell, so does not usually last all the way until 4 for rear facing. Joie Steadi lasts longer because of the headrest that moves up and down. Both of these have the same seatbelt path as an infant carrier, so the seatbelt is in the way, which is a bit of a pain in a 5-door. It's also worth noting that they don't have the improved side impact protection like R129 seats would have, and the seatbelt method of fitting requires you to use the reclined position always for rear facing. It's likely that these seats don't perform as well as a seat with support leg, although they are still better than forward facing. If you are likely to forward face in the seat later, I would not recommend them. Possibly the newer R129 versions are better, although then you're getting close to the isofix spin prices anyway. They are good if you need a portable/spare seat or you want to rear face on a very low budget.
If you are going to invest £400+ in spin seats, lastly, it's worth trying to work out what centile your LO is on and what age roughly they will reach 18kg / 105cm. If it comes out at a much younger age than you feel happy changing to a booster seat using just seatbelt, then you might need to look at 25kg+ seats longer term, or be aware that once the 18kg seat is outgrown, you'll be looking for a 25kg harness type seat, which is more expensive than just a booster seat.