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 olive-drab territory — that bronzed, greyed-off green 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 greyer next to the deeper, more bronzed 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 marble-dust pigment load and depth — especially in their Marble Matt finish — 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 olive greens 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.