Honestly? Dulux doesn't have a proper match for Little Greene::Basalt, and I'd not pretend otherwise.
Basalt is a deep, slightly green-grey off-black — there's real warmth and complexity in it. The closest Dulux gets is Dulux::Black (LRV 1.2), and even that sits at ΔE 7.8 from the original. Anything above ΔE 2.5 is a visible difference, so 7.8 is a genuinely different colour — Dulux Black is flatter, truer black, and lacks the smoky depth that makes Basalt worth choosing in the first place.
The other contender, Dulux::Slow Swing (LRV 3.2), is further still at ΔE 10. That's not a match by any measure — it reads as a different colour entirely. I wouldn't recommend it as a substitute for Basalt.
So here's my honest take: if you've fallen for Basalt, buy Basalt. This is exactly the kind of colour where a cheaper near-miss costs you the thing you liked. Off-blacks live or die on their undertone, and Basalt's green-grey character is the whole point — lose that and you've just got generic black on the wall.
Where Dulux genuinely earns its place is on the practical layers. Use Zinsser BIN or Cover Stain if you're blocking stains or going over something dodgy, and Dulux trade emulsion or eggshell is fine for a base coat build-up — but bring the Little Greene tin out for the finish coat that you'll actually see.
If budget is the real driver, I'd rather steer you to a different off-black that Dulux does carry well than fudge Basalt. But if it's specifically Basalt's smoky, slightly olive-black you want, no Dulux tin gets you there — order the real thing and don't look back.