Hi Little,
I'm the Reviews Editor here at Mumsnet and noticed your queries had gone unanswered.
The majority of highback booster seats are £150+. But if you're looking for a highback booster at a lower price point, a lot of Mumsnetters recommend the Recaro Monza Nova 2 Seatfix. It's £135 at John Lewis and uses Isofix. It also has some nice extra features, such as an MP3 pocket on the side with an input cable and speakers in the headrest.
The Joie Trillo or Joie Trillo LX come in at between £50 and £80 depending where you buy them. These two seats don't use traditional Isofix, but instead use Joie's custom ISOSAFE connectors, which attaches to your car's Isofix bars.
Meanwhile, Cybex seats (such as the Solution Q3-Fix at £190) are pricey, but include a headrest that reclines.
All these car seat options will last your child from 15kg / age 4 until they're 36kg / age 12 (approx 150cm).
With regards to easy seat belt clipping, some Mumsnetters report the mifold booster seat is great. It's suitable from age 4-12 and works by pulling the seat belt down to the child's height, as opposed to bringing the child up to fit the belt (as traditional boosters and highback boosters do). Because it's the size of an iPad (yes, really) it's also easier to fit multiple boosters in the back of a car if you have multiple children. Mumsnetters say it's very easy to use - children are able to clip the belt themselves, provided you're happy for them to do so. However, the mifold is not a high back booster, so it has no side impact protection and it doesn't use Isofix. It could be a good option for when you're travelling, or as a spare to keep in the grandparents' cars though. The mifold comes in at £50.
We know that car seats can be a bit of a minefield, so if you're still feeling lost, we'd recommend popping into a store like Mothercare or John Lewis who can talk you through the different options and even check which seats are compatible with your car. Good luck!