Little Greene Aquamarine is a soft, yellow-leaning sage green — a proper heritage shade with a gentle, faded quality that moves with the light. If you want to stay in the Dulux range, your two best bets are Tuscan Glade 3 and Dewy Lawn, both landing at ΔE 2.4 from the original with an LRV of 45.6.
A ΔE of 2.4 sits just inside the "very close" band (under 2.5). That means most people won't clock the difference on a finished wall, but a trained eye comparing two patches side by side in raw daylight might. It's a good match — not an imperceptible one. So if you're touching in next to existing Aquamarine, I'd be a touch cautious; for a fresh job on a clean wall, either Dulux option will read as the same colour to anyone who isn't holding the original card next to it.
Why two matches at identical figures? Because they're the same measured colour under two names — the library records the same colour for both, so don't go hunting for an undertone difference between them; there isn't one. Pick whichever your Dulux stockist can mix, grab a tester pot, and try it in the actual room — north light will pull this colour cooler and greyer, while a sunny south-facing room will warm it up and bring out the green.
The honest question: should you bother? If budget's the driver, the Dulux route makes sense. But Little Greene's pigment density and the way Aquamarine moves across the day is part of what you're paying for. If the colour matters that much, get an Aquamarine sample alongside the Dulux match and judge in situ before you commit.
Practical tip: at LRV 45.6 this is a mid-tone — it won't bounce light around a small room, so reserve it for spaces with decent natural light or pair it with a paler ceiling and trim to keep things lifted.