Your best bet from the Dulux range is Midwinter Tide, which lands at ΔE 2.3 from Stiffkey Blue. Anything under 2.5 is what we'd call very close — most people wouldn't clock the difference on a wall, especially once the light's changed through the day. Midwinter Tide has an LRV of 10.1, so it's holding onto that deep, inky character that makes Stiffkey such a cracking colour.
The runner-up is Breton Blue at ΔE 2.5, with a slightly lower LRV of 8.4. That makes it a touch darker and moodier — if anything it'll read more dramatic than the original in a dim room, so it's the one to lean towards if you want Stiffkey but deeper.
Here's the honest bit, mate: Stiffkey Blue is a glorious thing precisely because of how it shifts. In daylight it's a rich navy-blue; by lamplight it goes almost black with that moody undertone. Farrow & Ball get that from the depth and complexity of their pigment load, and no matched colour replicates the behaviour exactly — it matches the *colour*, not the *soul*. So if you're set on the genuine F&B look in a hero room, buy the real thing. If you're doing a hallway, a utility, or somewhere you want the navy hit at Dulux pricing, Midwinter Tide will do you proud.
One thing to watch: deep blues are unforgiving over patchy or stained walls. Whatever you go for, get a coat of grey-tinted undercoat down first — it lets the colour reach full depth in fewer coats and saves you a third coat of expensive topcoat. Dulux's own deep-base system works fine here.
Always, always test before you commit. Paint two coats on a bit of lining paper, tape it up, and look at it morning and night. Deep blues are the colour family most likely to surprise you between the tin and the wall.