I think the Two Way Elite is your best option really. There are a couple of cheaper forward facing seats which harness up to 25kg, but they both require top tether meaning they aren't especially universal in cars and you might struggle to fit them. If you only ever go in fairly new cars it would be OK.
The TWE is an imported seat and can be bought from the In Car Safety Centre - as far as I know they do have stock. It looks like you're looking at Tony Kealy's?
Other 25kg ERF under £200 are the original Britax Max-Way, (not Plus) the Diono Radian 5 (also folds, which may be good) and the Axkid Move (just under from one shop).
I would normally be cautious about buying seats second hand, but I do wonder if it might not be quite as risky with a 25kg ERF seat, given that there is a fairly reasonable assumption that anyone buying one of those in the first place has thoroughly done their homework in terms of car seat safety. Obviously it's your call, and you still wouldn't really know, but it might be a way of getting a longer lasting seat on a smaller budget which might be reasonable if you don't think you'll use it very much.
Alternatively you could look at high backed boosters with an anti-submarine strap such as the Britax Kidfix 2 (and higher models). You're looking at about £130-160 ish, but at least it would last you until no longer needing a car seat, and these types of seat are quicker/easier to fit and more portable than something like the Two Way Elite. The only(!) issue is that 2 is very little to be forward facing, let alone in a high backed booster seat, so you'd have to weigh up the relative risk with the alternative (not using a car seat at all in taxis?) and of course likelihood of actually having a crash if you seldom travel in cars.
You could also potentially try to avoid car travel over the next 6-12 months and put off the decision for as long as possible? Allowing time for more car seats to come onto the market (harness to 25kg seems to be a big market push at the moment), current car seats to come down in price, you to save, your DS to get a bit older.