Hi OP
At not quite 3 I assume you want your little one still in a harness? I wouldn't recommend putting them into a booster seat at this age. So it's just whether you want rear facing or forward facing.
Up to £200 you don't have a huge amount of choice for rear facing at the moment - only really Britax Multi-Tech 3 on amazon which is £190. It doesn't use isofix, because isofix has a weight limit on it that these larger seats don't meet. So they are seatbelt fitted, they also use what are called Swedish Style tethers, which go around the seat in front or the front seat rails. You could also look at Britax Two Way Elite which is cheaper but it's a much older, more basic seat design and some people don't like it.
If you can wait until Easter or even May Day (sounds like you have time to do this unless he's 17.9kg!) there might be some offers on the Britax Max Way or Axkid Move for around the £200 mark. One or the other of these usually goes on offer - the Max Way has only just gone back up so probably not that one. The next sale point after May is usually July.
These are specialist ERF seats so you'll normally need an ERF specialist retailer or you can buy online. I'd recommend going through the specialist if you can find one locally, as they can help you with fitting. If you can't find one locally, look for one offering remote assistance with fitting and if this is not an option, watch some youtube videos for fitting the seat you choose.
Or you could look at forward facing seats. For these you'll need top tether in your car - they usually fit with a combination of seatbelt, isofix and top tether all at the same time. There aren't a huge amount of 25kg harnessing FF seats on the market but they are a bit cheaper. The two I'd probably look at are Joie Bold or BabyAuto Dupla (make sure to get the isofix version, not the beltfit version - this is incorrectly listed in many places as harnessing to 25kg, but it's actually 18kg. The isofix one is 25kg.)
The quality and reputation of the forward facing 25kg seats is a bit more unclear than the rear facing ones which are generally highly tested and rated. But if you need FF for whatever reason and you need the harness, then they do at least exist! Cozy n Safe is the other brand that do them (Excalibur and Hudson) but I think the Joie/BabyAuto is the better choice. The complaints I've heard about them is that the Joie has a tight crotch strap which can bother children, especially boys, and the BabyAuto doesn't seem to fit securely in all cars. But both of them are fine for most people. Joie Bold is about £160 and BabyAuto Dupla is about £100-130 depending on the shop.