As for car seats, since LO is under 15kg she is legally not allowed to sit in a high backed booster seat using adult belt. She must still use a seat with 5 point harness or impact shield. The minimum for high backed booster is 100cm and 15kg, she must meet both. It's recommended not to go to HBB at the very minimum either but wait as long as possible so she is as close as possible to the maximum of the previous stage (which is 105cm/18kg, may vary slightly by seat.)
If her height is in proportion with her weight, I'd suggest one of the 105cm limit seats since these tend to get better test results than the Joie Bold.
Britax Advansafix IV R
Britax Evolvafix
Maxi Cosi Titan Pro i-size or Plus i-size
Silver Cross Balance
These are all nice seats to use with good crash test safety.
If it turns out you don't have top tethers
Recaro Tian Elite - best rated seatbelt fit 123 seat.
Britax Evolva 123 - cheaper but still good safety.
If she is taller than 105cm already you will need Joie Bold for the height capacity.
Between 100-105cm, the Britax seats are roomy (but go for non i-size).
If you have money to burn you could look at Cybex Anoris T. It's incredibly safe, fits up to 115cm but also about £700.
Note if you look at the Maxi Cosi Titan, there is an older version "Titan" "Titan Pro" "Titan Plus" "Titan i-size" which has a smaller shell and a short harness. The new versions which have both "pro" or "plus" AND "i-size" in the name have a larger shell and probably a longer harness. I would recommend testing this in a shop unless DC is well under 100cm.