You don't, there's an overlap in the groups. Group 0 goes to 10 or 11kg, and Group 0+ is up to 13kg.
So depending on what the weight limit is, you should be able to continue using the seat for longer. 9kg is the minimum weight that you can legally turn them forwards. You must not use the seat RF past the maximum weight stated, though, as it won't have been tested for this weight.
The "well known" brands like Graco, Maxi Cosi and Britax combination seats tend to be RF until 13kg, but the very cheap brands like Nania, Babystart, and some of the own branded ones are only until 10kg.
I would recommend the Britax First Class, it's RF until 13kg, and it has two belt routings which helps to avoid buckle crunch even when fitting in many different cars. You can usually find it on offer for around £100.
Other than this as a fellow non-driver, I really rate the Kiddy car seats with the impact shield as they're so light and easy to transport/swap between cars. They are more expensive though.