Graco Eldura and Joie Fortifi are combination harness and booster seats, but you don't need the 5 point harness, do you? As he is 125cm, he is also over the limit to use the harness in these seats, so there is no point buying a seat with that capability as it just means it is designed less with the booster stage in mind. Get a dedicated high back booster.
If you did want a harnessed seat for him (IMO this is unnecessary, assuming no developmental delay that would make it hard for him to understand the need to stay in a seatbelt) the only ones which would work are the old Joie Bold R44 or Cozy n Safe Hudson R44 because they don't have a height limit. Both of them need isofix and top tether for the harnessed mode, and it's recommended to isofix combination seats anyway in booster mode because they tend to be heavy.
In "pure" HBBs isofix is optional to use. The Britax Kidfix is a good seat, Boots have the slightly older version on offer at the moment for £150, which is a good price. Some parents do like the 4th point of contact in the Britax Kidfix seats (the "crotch strap") - arguably this is unnecessary if the lap belt position is good. This is the cheapest seat with this feature on the market at the moment.
The best thing to do is usually to take the child and either most used or most awkward car to a shop and try out different models (anything Britax/Maxi Cosi/Cybex/Joie/Avionaut/Axkid/Britax/Recaro) to see which get a good fit - you want the seatbelt nice and low on your son's hips or tops of thighs is also OK, and for it to be easy to buckle in - sometimes with isofix boosters, the base of it is so far across it's hard to reach the buckle easily, which is a pain. If he wants to be independent he might like to do the seatbelt himself, you want to ensure that the belt moves smoothly through all the belt guides when it's plugged in, so it allows the seatbelt to retain its self-tightening function as he moves around. Sometimes a booster with less prominent belt guards at the hips is easier to fit the belt around correctly (usually, both parts of the belt under the guard but check the manual) especially for a child, but the more prominent guards may position the belt better especially for a more slim build.
Almost all HBBs on the market are up to 150cm so I wouldn't worry overly about this. Again, it's combo seats which tend to have a lower limit. In some cars, you can't extend the headrest up to the max height anyway without hitting the ceiling which is another thing you might want to check if you want to use the seat to its max capacity long term.
If you did want a cheaper seat, I'd look at Joie i-Traver, as I think the extendable leg rest is a great comfort feature, and Cybex tend to have a few older models at cheaper prices which are still very good seats. Or Joie also have i-Trillo, which is extremely reasonable, where the isofix is on a flexible strap rather than a rigid metal fixing, so it is lighter for use in the other car. I don't think it's as comfortable for long term use as the i-Traver, but it's a decent option and what we ended up with as my husband pointed out the cup holders on the i-Trillo, and my son would then accept nothing else 