If you want Strong White on a Dulux budget, go for Dulux Love Letter Cf19. At a ΔE of 0.5 from the original, it's effectively indistinguishable on the wall — a ΔE under 1 means your eye won't pick up the difference even side by side. LRV sits at 75.5, which is bang in line with what makes Strong White tick: a clean, contemporary off-white with the faintest grey undertone that keeps it from going stark.
The runner-up is Dulux Swansdown at ΔE 1.2, LRV 75.7. Still very close — under 2.5 is what the trade considers a confident match — but Love Letter pips it. If your Dulux stockist has both on a fan deck, hold them against a sample of Strong White in your own light before committing.
Now the honest bit, mate: Strong White's magic isn't just the colour, it's the finish. Farrow & Ball's Estate Emulsion has a chalky, light-absorbing depth that Dulux's standard Vinyl Matt doesn't quite replicate. If you're matching for cost, Dulux Heritage Velvet Matt gets you closer to that flat, luxurious look than the trade vinyl does — worth specifying if the room's a feature space.
The other thing to flag: Strong White shifts. In a north-facing room it leans cool and grey; in warm afternoon light it softens and reads almost creamy. Whichever Dulux match you pick will do the same dance, because that's the colour doing it, not the brand. So test in the actual room, at the actual time of day you'll use it most.
Practical steps: buy a Dulux Love Letter Cf19 tester, paint two coats on lining paper, and prop it against your existing Strong White (or a sample pot) on the wall it's destined for. Live with it for 48 hours across morning and evening light. If it holds, you've saved yourself a few quid per litre without anyone being the wiser.