Personally I'd go for two speakers; producing on a single speaker just isn't fun, so even if it arguably it is good for accuracy, I wouldn't go there. And even if you want to work in mono cos it's more jungle, it's less involving thing when it's just coming at you from one speaker all day...
Budget precludes a sub I think, so you're looking at 2.0
Evenness of bass response will be your issue in non-ideal room (heck, even in a half decent room, it's always the biggest anomaly, and in basic terms you can easily end up mixing some bass notes higher or lower in level than you intend)
So, if you've got a decent mic (or can borrow one) and are up for doing some googling about room correction and getting familiar with REW, I'd probably say getting a MiniDSP 2x4 is a better investment than spending that extra £100 on monitors. While the 2x4 is hardly hardly last word in fidelity, for production, a more neutral frequency response is almost certainly worth the sacrifice in purity.
You could do that basic room EQ in the monitor chain of your DAW of course, and not spend that £100, but it just becomes a ball ache and settings are easily lost or neglected etc. Then you're also stuck having to play everything through the DAW. You can't just spin in an external etc... So anyway, I always put my "room correction" in hardware for simplicity.
Anyway, my recommendations would be:
1x MiniDSP 2x4 (@ £100)
Plus one of....
2x ADAM T7V
2x KALI LP-6
2x JBL 306
2x Mackie MR624
They're all "good" speakers and all about £300/pair.
To my ear, the Mackie, Kali and JBL all have a (relatively) similar character, the ADAM is the one that sounds "nicest" and has great LF handling for it's size, but might be a bit too "sweet" sounding for jungle production? It sorta sparkles but doesn't shove midrange at you like the others.
If your room is untreated I'd *guess* the LP6 or 306 might be good options as their off-axis is "good" so the reflections aren't too colored - although you could search ASR to confirm. Of those two the LP6 probably has the better designed bass port which allows you to play louder & cleaner IMHO.
Lastly, having waffled on, to directly answer your question if you really don't want to get involved in all the room EQ carnage; the 8-inch versions of the above I.e. JBL 308 or Kali LP8 will give you a tiny bit more LF extension for your £400.
I've given up buying 2nd hand at this price point. Firstly, speaker design moves on, so older speakers are on the back foot. Secondly, particularly with studio monitors, they might have seen some abuse (or rather, high SPL accidents that happen in the studio in a way that rarely do in the living room!) and even if you collect them in person, that damage doesn't always come to light in a quick listening check.
That's my thoughts anyway. Hope that helps!?