Joie Trillo is much better than Graco. Joie generally have a great ratio for price to safety/quality. They're not luxury goods, but unlike some budget brands they don't compromise on safety, which is obviously what you want in a car seat.
Isofix anchors in this type of seat just keep the seat in place when it's not strapped in, which is useful for when DC aren't in car (safer for driver/adult passengers) and prevents it tipping and moving as they climb into it and obscuring the buckle. Doesn't make a huge difference in safety for the seat although an isofix secured one will have less sideways movement if you were hit side on.
Maxi Cosi are still good for boosters - look at the Rodifix. Recaro also do a good booster, but it's over £100 so not so good on a budget.
Diono is a little known brand which are really good for boosters - Cambria 2 is a reasonably priced one of theirs. The good thing about these is they are very tall.
Cybex are another good quality brand - they have a couple around the £60 mark if you find some in the sale.
Definitely avoid Nania, generic/own brand and the Disney/superhero branded ones as these are no good.
I'd recommend you try out in car if possible. You're looking for good contact with the seat back and good movement of the seatbelt (it should automatically retract, not get stuck.) If he's on the older side of needing a booster I'd also definitely recommend you check what the max shoulder/headrest height is, as some are fairly low down meaning children grow out of it fairly soon. You want lots of room to expand. Also obviously check with child as to whether it's comfy :)
You get a better fit with most booster seats if you remove your car's rear headrest and either reinsert backwards or just take it totally off. You want the booster seat to be flush with the seat back as much as possible.