Mylands paint is gorgeous stuff — proper depth, lovely sheen — but it's premium priced, and Brompton Road No.205 is a rich, moody green that you can get reasonably close to for less.
The nearest match is Dulux Forest Shade (LRV 11.3), measuring ΔE 3.2 from the original. That's not imperceptible — you'd spot the difference if you painted a panel of each side by side — but in a real room, on its own four walls, you'd be hard pressed to call it out. Dulux is widely stocked and trade-priced, so it's the sensible budget pick.
Close behind are Crown Woodland Wanderer (LRV 10.7) and COAT Nomad (LRV 13.9), both at ΔE 3.6. Crown's another easy-to-find, keenly-priced option. COAT sits a touch brighter on the LRV scale, so Nomad will read very slightly lighter and a hair fresher on the wall — worth knowing if your room's already short on daylight.
Now the honest bit: at ΔE 3.2-3.6, these are *good* matches, not exact ones. The thing you lose with Mylands isn't really the colour — it's the finish. Mylands' Marble Matt and their eggshells have a particular soft, chalky-but-tough quality that the cheaper brands don't quite replicate. So if it's purely the shade you're chasing, save your money. If it's the *feel* of the paint underfoot — the way it sits on woodwork especially — that's harder to dupe.
My advice: order tester pots of all three, paint big A2 sheets, and live with them for a couple of days. Deep greens shift enormously between daylight and lamplight, and the winner in a north-facing room won't be the same as in a bright south-facing one. Whichever you land on, two coats over a grey-tinted undercoat will give you the truest depth — deep colours always benefit from a primer toned towards the topcoat.