If you want Sanderson Newby Green in a Dulux tin, go for Dulux Breton Blue. It lands at a ΔE of 1.3 from the original — that's well inside the "very close" threshold and, on a wall, you'd struggle to tell them apart. The name throws people, mind: Newby Green is one of those deep, moody greens that reads distinctly teal-to-blue in lower light, which is exactly why Dulux's nearest neighbour sits in the "blue" camp.
Worth flagging the LRV here too. Breton Blue comes in at LRV 8.4, so it's a genuinely dark colour that soaks up light. That's the whole point of a shade like this — it's built for drama on a chimney breast, a panelled dining room, or a small cloakroom you want to feel like a jewel box. Just go in with eyes open: in a north-facing room with little natural light, it'll feel properly inky.
The runner-up is Dulux Night Seas at ΔE 2.8 and LRV 7.9. That's still a respectable match, but it's a touch further off than Breton Blue and reads marginally cooler. If you can only get hold of one, Breton Blue is the one to chase.
The usual "but what about" question: will a Dulux match in the same depth as a Sanderson colour behave identically? Not quite. Sanderson's own formulations carry their pigment and finish character, so even at ΔE 1.3 the sheen and the way it shifts through the day can differ slightly between brands. That's normal and nothing to worry about.
Practical advice: never sign off a dark colour off the fan deck. Get a sample pot of Breton Blue, paint two coats onto a bit of lining paper or an offcut, and move it round the room across the day — morning, midday, and under your evening lamps. Dark teals like this are completely transformed by artificial light, so the lamp test matters more than the daylight one.