Yes the Britax ones are on offer at the moment, so you could look at those :)
OTOH you could hang tight. He's close to 4, so not a disaster to forward face and you still have 1.5kg to go on the seats that you have, which isn't nothing. Since they were old reg Group 2, I don't think that the 25kg will be a hard limit of their capacity - obviously I would not go massively over this, because you would be operating in untested waters at that point. But for example, Klippan used the exact same shell of seat for the Century (R44, 25kg limit) and Opti (R129, 32kg/125cm limit) and there are a few FF seats which are identical other than the certification as well. I have Britax Advansafix i-size which has the bizarre wording in the manual that if it's used in an i-size car position then it's certified up to 102cm and 21kg, but if it's plugged into isofix ie exactly the same fitting method but the car is pre-2013 so it's not an i-size certified position, then it's allowed to use up to 18kg with no height limit mentioned. 🤷♀️
So I don't think you need to be weighing him every day in case he ticks over to 25,001g either, if that makes sense. You could just weigh him intermittently and if you notice one day oops he's 25.5/26 kilos now - then it's time to buy high back booster seats.
At this age they really can hover at a specific weight for a while anyway, as in over a year sometimes. Their growth seems to go in total spurts and they seem to either grow up or out, rather than both at once slowly as you'd probably assume. (I swear my 6yo has been 116cm for over a year now). Assuming the measurement was from autumn, it might also be that he has recently had such a growth spurt in the form of brown fat laid down for winter. (Even though we have central heating we are still mammals and still do this.)
There aren't any forward facing 5-point harness seats which go over 25kg unless you look at special needs restraints or import a seat from another country (which isn't technically legal to use in the UK). So once you switch to FF it will likely be a high back booster. If this makes you nervous, it might be worth looking at the seats with the extra restraint point on the lap belt e.g. Britax Kidfix, Besafe izi Flex fix. Honestly, it's not really necessary but they are nice seats anyway - and a lot of parents like it as a feature esp if feeling DC is still quite young. I believe the Britax Kidfix is on a Black Friday offer if you wanted to get some to keep at home in case a quick changeover does turn out to be needed.