Right, let's be straight with you: there's no Dulux colour that truly lands on Little Greene Marine Blue. The two nearest options are Dulux Azure Fusion 1 (LRV 5.4, ΔE 6.5 from the original) and Dulux Hawaiian Blue 1 (LRV 7.7, ΔE 6.8). For context, a ΔE under 1 is imperceptible and under 2.5 is very close — so a ΔE of 6.5 is a noticeable difference, not a clone. You'd see it if you held both up side by side.
Marine Blue is a deep, slightly inky teal-leaning blue with real character — the kind of colour Little Greene does so well. Of the two Dulux options, Azure Fusion 1 is the closer of the pair and the slightly darker, with an LRV of 5.4 against Hawaiian Blue's 7.7. If you're after that dramatic, swallow-you-whole depth, Azure Fusion 1 is the one to test first.
Now the obvious question: should you bother colour-matching at all? If you've fallen for Marine Blue specifically, my honest advice is buy the Little Greene. A ΔE of 6.5 means you're compromising on the very thing that made you want it. Little Greene's Intelligent and Absolute Matt emulsions also have a depth and pigment richness that's hard to replicate, and at this kind of saturation that quality really shows.
Where Dulux makes sense is budget — covering a large area, a rental, or a job where the exact undertone matters less. In that case go with Azure Fusion 1.
Whatever you choose, order a sample pot and paint two coats on lining paper, then view it on different walls and at different times of day. Deep blues shift hugely with light, and a dark north-facing room will read very differently from a sunny one. Live with it for a few days before you commit, and you'll get it sorted properly.