Hey all...
I'm a child seat fitter for Halfords and I don't believe there is a group 1-2-3 isofix seat. I've been looking for a while too since a customer requested one. I am not sure why this is, it may be due to the need for deep side impact protection for younger children whilst retaining the booster part to enable the seat to be suitable to the age of 11/12.
While Isofix is the absolute safest option for your child and there should be no compromise on this, it is equally unsafe to keep them in a seat they have outgrown due to their height or weight. If your child has reached 15kg's or the back of their head is at the top of their group 1 seat then they are usually able to move up to a group 2-3 seat. I WOULD NOT put a child of 15kg's (approx 4 yrs) on a booster seat yet - go for a high back booster which you can get with isofix. The car seat belt will need to be level with the child's shoulder.
It is a good idea to take your child with you when buying a new seat as you can check they are suitable for it (giving the child's age at this stage is pointless as every child is so different in height and weight) For a group 1-2-3 seat I would recommend the following (all seat belt fitted)
Generally Best Price - The Graco Nautilus or Britax Evolva 1-2-3
Comfort and safety - Recaro Young Sport
Last pointer - I personally NEVER recommend a group 1-2-3 seat for a child of 9mths/9kg's. When your child is first going forward facing they need to have a seat with a decent recline on it to provide support to their spine and neck. If you have an ISOFIX compatible car and can afford an ISOFIX seat then get one! If not go for a group 1 seat with a seat belt tensioner - such as the Maxi Cosi Priori XP or the Maxi Cosi TOBI.
Also...always check that your car is safety tested for the isofix seat with the manufacturer. Just because your car is isofix doesn't mean it'll immediately be a safe fit (I've had alot of people be suprised at this when advising on seats so thought i'd best add it!)
Hope this is helpful!