If you're after Holland Park No.5 in a Dulux tin, Dulux Vanilla Mist 1 is your colour. It lands at ΔE 0.6 from the Mylands original, which is well under the ΔE 1 threshold — meaning the two are effectively indistinguishable side by side. You'd not pick them apart on a wall. Its LRV of 80.8 keeps it in that soft, light, warm-neutral territory that makes Holland Park such an easy room colour.
Close behind is Dulux Natural Taupe 4 at ΔE 0.8, with an LRV of 82. Both are very close indeed, so the choice between them comes down to which Dulux product line and sheen you want rather than any meaningful colour difference. If you're decorating a whole room and want the absolute nearest hit, go Vanilla Mist 1.
Now, the honest caveat: Mylands paint and Dulux paint aren't the same beast. Mylands uses a higher pigment load and tends to have a richer, more characterful depth in changing light. A colour-match gets you the *hue* spot on, but you may notice the Mylands version shifts a touch more subtly through the day, especially in a north-facing room. The match is excellent; the finish character is brand-specific.
So my advice — if budget is the reason you're switching, Dulux Vanilla Mist 1 will do you proud and nobody will know. If you've fallen for Holland Park specifically because of how it *behaves* on the wall, it might be worth buying a single Mylands tester pot alongside the Dulux and brushing both out on a bit of lining paper. Move it around the room, look at it morning and evening, then commit.
Always test the actual paint, not a printed swatch. Two coats, left to dry properly, viewed in your own light — that's the only test that counts.