Hi OP, is it the Radius or the Solar you're looking at?
The Radius is a rear facing and forward facing, 360 degree spin seat but it does not go up to 12 years, it's a Group 0/1/2 which means that it goes up to 13kg rear facing (about 2 years), 18kg forward facing (about 3-4 years) and then it's a high backed booster seat up to 25kg (about 7 years). In the UK children need to use a car seat until they are 12 years old or 135cm, so you will need to buy another car seat after this one.
The Solar is a forward facing only seat which uses a 5 point harness up to 18kg (about 3-4 years) and then is a high backed booster seat up to approx 12 years. However this seat would not be a good idea for an 8 month old, even if they are already 9kg, because forward facing is still very risky at 8 months old. You want to aim for a bare minimum of 15 months rear facing, but it is best to rear face for as long as you can. Even if you did decide to forward face now, I would not say that this type of seat is a good option for children just entering the 5-point harness stage. They are sometimes useful for children who are getting to the end of the 5 point harness stage, who you don't want to just buy a booster seat but don't want to buy a seat that only lasts another 6-12 months either. When they are just starting in this sitting up harnessed stage (coming out of reclined baby carry seat) then you want to go for a seat that just covers the harness stage and doesn't convert to booster seat. Ideally rear facing, but also important even if you go for a forward facing seat.
The "Radius" type of seat which covers all the different stages are popular at the moment because the promise of lasting the whole time you need a car seat sounds really great. In practice, though, they tend to be a bit of a pain to use and have several downsides compared to seats which cover just one, maybe two different stages. They also tend to be a false economy - for example, the Radius costs £189 according to their website, but given you are essentially getting a very basic spin seat plus a high backed booster that only lasts until 25kg, you would do better to get something like Graco Turn2Me as your spin seat, which is about £150, and then in 4 years' time buy a basic high backed booster for £40, which will fit better and last you to the proper 150cm/12 years. Same money spent, and an overall much better quality option.
The Graco is nicer to use, the build quality is better, it can rear face up to 18kg, and any dedicated high backed booster (especially from a good-but-cheap brand, like Joie) will fit better than the Radius in booster mode. I notice there are no pictures of any child model in the booster mode - I think this will be because where they have the belt guides on the shell, like that one does (see the red guides on top), this is a very outdated style and tends to result in poor belt fit compared to booster seats which have the belt guide in the headrest (e.g. Joie Trillo), because the seatbelt can't move up as the child grows. This will mean that it is placed too high for younger children and too low for older children. If you do get this seat I would not recommend using the booster mode at all, and in that case if your budget is approx £190, it would be worth comparing with:
Joie 360 Spin - nicer spin seat, rear facing up to 18kg / forward facing to 18kg.
Britax Dualfix 2R - nicer spin seat, anti-rebound bar, RF/FF both to 18kg
Britax Max Way - very safe extended rear facing seat up to 25kg.
Joie Stages - much cheaper, RF to 18kg, FF to 18kg, 25kg booster seat (better belt fit).
