If you're after Portland Stone in a Dulux tin, your two best bets are Dulux Fresh Artichoke (LRV 51, ΔE 1.9) and Dulux Crushed Aloe (LRV 50.8, ΔE 1.9). Both land at ΔE 1.9 from the original, which is within the "very close" band — most people standing in a room wouldn't clock the difference, though a careful eye in raking daylight just might.
Why two matches at the same ΔE? Because they're the same measured colour under two names — identical on the meter, with only a rounding hair between the recorded light values. Don't go hunting for an undertone difference between them; there isn't one. Pick whichever your Dulux stockist can mix without a fuss.
A word on what you're trading. Portland Stone is one of Little Greene's lovely soft greened-stone neutrals, and part of its appeal is the depth and pigment quality of the paint itself, not just the hex value. A ΔE 1.9 Dulux match gets you the colour very close, but Little Greene's Intelligent Matt and Absolute Matt have a particular flat, chalky richness that a Dulux finish won't replicate exactly. If it's the colour you're chasing for a big job and budget matters, the Dulux match is a sensible call. If it's the finish and feel you fell for, sample the Dulux against the original before you commit.
Practical advice: buy a tester pot of either name — they'll mix out the same — paint two coats on a bit of lining paper, and tape it up next to a Portland Stone sample. Live with it for 48 hours across changing light. Cheaper than repainting a whole room because the match looked right on a 5cm chip in the shop.