Go for Dulux Tawny Owl. At ΔE 2.2 from Mylands Messel No.39 it sits comfortably inside the "very close" bracket — under 2.5 means the difference is barely perceptible once it's up on a wall and dry. Tawny Owl has an LRV of 10.3, so it lands in the same deep, muddy taupe-brown territory that makes Messel such a moody, sophisticated choice.
The alternative is Dulux Wild Mushroom (LRV 13), but at ΔE 3.5 it's a step further away. That gap is visible — Wild Mushroom reads lighter and a touch greener-grey next to the warmer, earthier Messel. If you're trying to genuinely replicate Messel, it's not the one. It's only worth considering if you actively want to lift the room a shade.
Now, the honest bit: a colour match gets you the hue, but it won't give you the Mylands paint. Messel No.39 is part of Mylands' premium line, and their pigment load and depth — especially in their Marble Matt and FSC-certified finishes — carry a richness that's hard to fully clone in a mainstream trade emulsion. On a dark, characterful colour like this, that depth shows. So if budget allows and the room's a feature space, I'd stick with the original Mylands.
But if you're spec'ing for a rental, a big square-footage job, or you simply want Dulux's availability and price point, Tawny Owl will do the job properly. It's the right call.
Practical advice: test before you commit, every time. Order a sample of Tawny Owl and, ideally, a Messel sample pot too, paint generous patches side by side, and view them in daylight and lamplight. Deep browns shift a lot under artificial light, and ΔE figures are measured under standard conditions — your room isn't a lab. Two coats over a tinted grey primer like Zinsser's will give you the truest read of the finished colour.