There's no perfect Dulux twin for Inchyra Blue — but the two that get you nearest are Dulux Smokey Slate (LRV 12.3, ΔE 3.2 from the original) and Dulux Fresh Slate (LRV 13.7, ΔE 3.4).
A word on those numbers. A ΔE under 1 is imperceptible, under 2.5 is very close. At 3.2 and 3.4, both of these are *visible* misses if you held a Farrow & Ball card next to a Dulux one — close enough that nobody walking into the room would clock it, but not so close that I'd promise you a seamless touch-up over an existing F&B wall. They're the best Dulux has, not a clone.
Why the gap? Inchyra Blue is that brilliant slatey blue-green-grey that shifts depending on light — reads deep teal in shadow, dusty grey in flat daylight. It's a hard colour to nail because so much of its character is in that chameleon quality, and Dulux's slates land a touch more straightforwardly grey-blue. At LRV 12-14 you're firmly in dark, moody territory either way, so all three will behave similarly in terms of how much light they swallow.
The "but what about" question: if you genuinely love Inchyra Blue, why match it at all? The honest answer is usually budget or trade availability — Dulux is easier to get tinted in bulk and cheaper per litre. Fair enough. But if it's a feature wall or a small room where the colour is the whole point, I'd just buy the real thing. The F&B depth and pigment load earns its keep on a hero colour.
Practical advice: order both Dulux Smokey Slate and Fresh Slate as sample pots, paint big A2 patches alongside an Inchyra Blue sample, and judge them in your own light morning and evening. On a north-facing wall the cooler one will pull greyer — let the room decide.