Messel No.39 is one of Mylands' deep olive-drab greens — a bronzed, greyed-off green with real depth — and lovely though it is, you're paying a premium for that velvety finish. If it's the colour you're after rather than the brand, you've got a genuinely good cheaper option.
Dulux Tawny Owl (LRV 10.3) is the standout. At ΔE 2.2 from Messel it's very close — close enough that on the wall, in normal light, you'd be hard pushed to tell them apart. Order it in the Dulux Heritage range for a properly flat, deep finish, or in Dulux trade emulsion if you want to keep costs right down. Either way you're saving meaningfully against Mylands list price.
COAT Rocky (LRV 9.9) is the next nearest at ΔE 3.5 — a touch further off, so expect a visible difference if you hold them side by side, but still a handsome deep shade in its own right, and COAT's quality is excellent for the money. If you fancy a low-VOC, quick-drying paint and you're ordering online anyway, it's worth a look.
Give Crown Bitter Chocolate a miss for this particular job. It's a perfectly decent paint, but at ΔE 8 it's a different colour from Messel — warmer and more obviously chocolate-brown where Messel has that greyed-off olive depth. You'd notice the swap.
The one thing to remember: Mylands' appeal is partly the finish, not just the hue. That deep, chalky-but-rich surface is part of why people pay for it. A Dulux match gets you the colour, but if you're doing a feature wall or panelling where the sheen and depth really carry the look, sample both side by side before you commit.
Buy a tester of Tawny Owl, paint a decent patch — A4 minimum — next to your Messel reference, and view it morning and evening. Deep olive greens like this shift a lot with light. If the patch holds up across the day, you've found your saving, mate.