Gosh, he's big for two! I think you need to be looking at an extended harness seat unless you're happy for him to be in a booster by three, because if he continues the way he's going, he'll have outgrown Group 1 before he is three years old. He'll hit the weight limit at about three and they normally outgrow in height before then. That means even the combination booster with harness seats won't be any good because the harnesses are normally only designed for up to 18kg and a little shorter than the average height at 18kg. Also, isofix is only approved up to 18kg, so if you get a seat which harnesses later, most of them are not isofix any more (or dual fit isofix+seatbelt).
So you need to look at seats which will harness up to the next weight category which is 25kg. This might be harder to find. Do you mind saying what area of the UK you're in? If you're near any branch of the In Car Safety Centre they would probably be able to help you, they are experts at this kind of thing.
What seat is he in now and how does he fit into it?
From a quick look it looks like your options are the Britax Advansafix, which, even if it's not officially approved, might be worth trying it out to see how it fits, the Britax 2 way Elite, and the Diono Radian 5. I can't see any others which harness to 25kg and are forward facing.
There are the BeSafe iZi Plus or the Britax Max-Way, harnessed to 25kg but only rear-facing which you might not want (though it is safer). Concord Reverso plus is RF only, harnessed to 23kg/105cm (weird mix there).
Lastly there are some isize seats which are approved up to 105cm, which is likely to be a bit higher than the normal Group 1 height range, and the weight limits are slightly higher too: 18.5 or 19kg. That's the Nuna Rebl, Maxi Cosi two way Pearl, Joie i-anchor, Cybex Sirona M2 i-size.
If you end up with a seat which only harnesses to 18 or 19kg and you need to look at a booster in a year's time, it would be worth looking at the Britax Kidfix 2 - because this one has a special extra feature which prevents young children from submarining under the seatbelt.
Unfortunately, none of these seats are cheap 