Go for Dulux Heritage Linen. It lands at ΔE 1.4 from Edward Bulmer Portland, which is well inside the "very close" band — under 2.5 ΔE is the point where most people can't reliably spot a difference, and 1.4 is genuinely tight. With an LRV of 72 it holds onto that soft, light, slightly earthy character Portland is known for, so you keep the airy feel without straying into stark white.
If DH Linen isn't to hand, Dulux Elderflower Tea is your backup at ΔE 1.6, LRV 72.4. Practically identical brightness, a whisper warmer. Both are bang on — I'd pick whichever finish and tin size your stockist has, because at this distance the colour difference is academic.
Now, the honest caveat. Edward Bulmer paints are natural, mineral-based formulations with very particular pigments, and they have a depth and chalky, breathable quality on the wall that no mainstream emulsion replicates exactly. A ΔE match measures colour, not soul. In flat daylight you'll get a faithful Dulux stand-in; in low or raking light, the genuine Bulmer article tends to shift and glow in a way DH Linen won't quite echo. If that quality is the whole reason you fell for Portland, it might be worth paying for the real thing on a feature wall and matching elsewhere.
For everything else — ceilings, hallways, the second coat on a big job where budget matters — DH Linen does the work beautifully. Dulux Heritage also gives you a proper flat matt and an eggshell in the same shade, which is handy if you want trim and walls to read as one tone.
Practical advice: never trust a chart or a screen for a near-match like this. Order a sample pot of DH Linen, paint a decent patch — at least A4 — beside your Portland reference, and check it morning and evening before you commit. At ΔE 1.4 you'll almost certainly be sorted, but light is the final judge.