I'm not hugely up to date with HBBs I must admit. But I do like the features of the Kidfix a lot, and I think it's one of the best performing in safety tests. I also found with the shape of the Britax headrests the child's head is contained very well if they fall asleep, which is rare at booster age but useful for long journeys. Cybex also have a reclining headrest for this reason, and score well for safety.
Diono seem to make very roomy/tall ones if that is a concern. Some children are too tall for standard/shorter high back boosters by about 8 years old and hence lose the side protection.
Joie Trillo LX is a good "sweet spot" for price/safety.
Isofix is quite useful if your child is likely to want to do up their own seatbelt - DS1 was always climbing in his which shifted it over the buckle and then he couldn't find the buckle to do it up, but didn't want help 
Isofix eliminates this problem, but adds about £50 to the price. However if you need to fit three across it can be a bit too rigid so better to find a seat without.
Worth noting you get a better booster fit in many cars if you either remove the (adult) headrest on that seat, or remove it and reinsert it backwards - you're aiming for the booster seat to be flush with the seat back, without a gap.
Best thing to do in any case is drive to a shop that stocks several boosters and try them out with your child and your car. You're looking for the seatbelt to cross at the child's collarbone and then cross their hips or tops of thighs, rather than sitting across the soft tissue of the abdomen. And of course that the child finds the seat comfortable and no part of it annoying. For the car, you want to check the belt retracts freely through the booster's belt guides e.g. if child leans forward and then sits back that the seatbelt correctly retracts, check that there is good contact between the child seat and the car seat, and that the plastic buckle isn't resting across the plastic of the booster seat - only the seatbelt should be touching here. It's not a bad idea to try the seat out at different headrest heights as well to check everything will still work well as your child grows.
If you can't find one in stock that you like e.g. the Britax ones linked, try another from the same brand if they look a similar shape from the website. Often brands have varying features on the same shell of seat, so you can do the fit check with any model.
Nania seats tend not to perform very well in safety testing and the own brands (including Cuggl) are almost always Nania rebranded, so I would avoid those. Possibly Graco as well, although I think their newer seats are OK.