If you want Threadneedle No.262 in a Dulux tin, go for Velvet Truffle 5. It sits at ΔE 0.5 from the original, which is below the threshold where the human eye can spot a difference — paint them side by side and you'd struggle to tell them apart. Its LRV of 64.9 keeps it in that soft, light-mid territory, a warm putty-stone that does the same gentle job as the Mylands.
The runner-up is Roman Stone 4 at ΔE 0.7, LRV 63.5 — still a very close match and a fraction deeper underfoot. Both are well within the "very close" band (anything under ΔE 2.5), so honestly either will do the business. I'd lean Velvet Truffle for the tighter figure, but if you're already buying Roman Stone for something else, don't lose sleep over the difference.
Now, the honest bit. A tight ΔE tells you the colours read the same in a controlled measurement, but it can't promise identical behaviour on your wall. Mylands and Dulux use different bases and pigment loads, and Mylands tends to have a richer, slightly chalkier depth in its mid-tones. In Dulux's Matt or Diamond Matt, the closest match may sit marginally flatter. The colour will be there; the *character* might be a touch different in raking light.
So why switch in the first place? Usually cost and availability — Dulux is everywhere and cheaper per litre, which matters on a big job. If that's your driver, Velvet Truffle 5 is the sensible call.
My advice: don't trust the swatch card or the ΔE alone. Get a sample pot of Velvet Truffle 5, paint two coats on a bit of lining paper, and tape it next to your Threadneedle reference in the actual room. Look at it morning and evening before you commit. If the finish matters to you, specify Dulux Heritage Velvet Matt rather than standard trade Matt — it's got more depth and holds the colour better.