Honest answer first: there's no perfect Dulux match for Farrow & Ball De Nimes. The nearest you'll get is Dulux Denim Drift, which measures ΔE 5 from the original. For context, anything under ΔE 2.5 is what I'd call very close and under 1 is imperceptible — so ΔE 5 is a noticeable gap, not a swap-and-forget. It's in the right family (that soft, slightly grey-blue denim territory) but it reads a shade brighter and cooler than De Nimes, which has more grey and earth pulling it back.
The other contender is Dulux Midnight Garden at ΔE 5.7. It's deeper and a bit moodier, with an LRV of 21 against Denim Drift's 26, so it'll feel heavier on the wall. If your room is south-facing and floods with light, Midnight Garden holds up better and won't bleach out. North-facing or dim, Denim Drift keeps things from going gloomy.
But here's the thing — if you genuinely love De Nimes, the honest advice is to just buy De Nimes. It's one of Farrow & Ball's best blues precisely because of that complex grey-green undertone that a mass-market match flattens out. Denim Drift is a perfectly nice blue, but it doesn't have the same depth, and at ΔE 5 you'll see the difference if the two ever sit side by side.
Where a Dulux match makes sense is if you're painting a big space on a budget, or matching existing Dulux trade work elsewhere in the house. In that case, go Denim Drift and don't look back.
Always test before committing. Get sample pots of both Dulux options and the real De Nimes, paint them on a bit of lining paper, and move them round the room across a full day. Blues shift dramatically with light, and that's where you'll see whether the compromise is one you can live with.