In general I would not recommend trying to combine too many stages into one, so not getting a seat which has the booster mode included as well. It makes for a very high up, heavy booster when the time comes, the seatbelt positioning isn't great on most of the combo boosters, and it's better all around IMO to just get a separate one, which also won't have had snacks ground into it for 3+ years (everyone says they won't feed the kids in the car; it doesn't last.) HBBs are coming down again in price and it's easy to find a decent one under £100 now.
Spin option is genuinely useful with toddlers, especially when you can't just get them into the seat in the house as you can with the baby seats. It makes it much easier to check the straps are all even and correct and at the right height etc. However with a high up car, bear in mind the extra lifting to get the child into the spin and it means they are less able to climb into the seat on their own.
Joie seats are good for the price, however if you have £300 to spend then you don't need to stick with Joie. You could look to import the Joie i-Soren (carseat.se have it) which lasts longer than their spin seats, up to 125cm/22kg, and costs under £300. This is an excellent option for the price.
For a Joie spin seat you only really need to spend £150-200 - look at the i-Pivot, the i-Spin 360 or the Graco Turn2Me i-size (which is basically the same seat as Joie Spin 360 GTi).
You don't need to ever forward face with harness if you don't want to. If you're happy to RF up to age 4ish then you can go straight from that to a high back booster.
Other seats for a £300 budget which are worth looking at IMO are the other spin seats (Cybex Sirona Gi i-size, Britax Dualfix M Plus, Maxi Cosi Mica)
or you have the ERF seats up to 125cm - these aren't usually isofix, but should benefit if you have a taller child because they last longer - every other seat mentioned (except Joie i-Soren) has a 105cm height limit. If you follow your baby's centile line you can roughly estimate what age they will reach 105cm. Plus with being lower to the seat, they are much easier to get children into, they have much better leg room than the Joie i-Soren (particularly the Axkid) and toddlers can often climb in themselves from age 2 or so. Easy to install in Volvos, as they tend to have tether loops in place already.
Up to £300 you could look at Axkid Minikid 2, Avionaut Sky Q, or the Britax seats SafeWay M or MaxSafe Pro.
If you like the idea of a spin but also the 125cm limit, you could also look at the Avionaut Stardust. It's a bit overbudget, but I thought I'd mention given that it is really the only seat with both these features. It also has the high back booster mode which it converts to later on. I have seen some ERF experts review it and they seem happy with it in HBB mode, so it's possible that my general dislike of combining HBB with other seat types is wrong on this occasion. (I haven't seen it IRL to say).
I would also say possible reccomendation for Britax Swivel - it has the booster mode built in, but due to this it seems to have a very tall allowance for the child's shoulders even in the harnessed mode. I don't know how it performs as a HBB, but it may be worth keeping just for the RF/FF stage because of this extra shoulder height. I understand that the height limit of that mode is still limited to 105cm - I've also seen Britax say it's OK to keep using the harness if the child fits into it and they are under the weight limit. That is extremely anecdotal, and probably not an official line.
Anyway - sorry, that is quite a lot of seats to look at, but hopefully something there is helpful.