Empire Grey No.171 is one of Mylands' lovely warm-leaning greys with that soft green undertone, but Mylands sits at the premium end and you don't always need it. If you're after the look without the spend, Dulux Bleached Lichen 1 is your best bet — it lands at ΔE 2.3 from the original, which is very close indeed. To the eye, in most lights, you'd struggle to tell them apart on a finished wall. And Dulux trade emulsion will cost you a good chunk less per tin.
If low-VOC matters to you — nursery, bedroom, someone in the house sensitive to smell — go for COAT Cold Brew (LRV 32.1) instead. It's a touch further off at ΔE 3.4, so a small but noticeable shift, but COAT's a genuinely good water-based paint with a clean finish and zero faff on application. Worth the slight compromise if eco-credentials are the priority.
I'd steer you away from Crown French Grey here. It's a perfectly nice colour in its own right (LRV 38.5), but at ΔE 4.5 from Empire Grey it's a clearly different shade — more obviously cool and lighter on the wall. Specify it if you actually like *it*, but don't buy it expecting an Empire Grey dupe, because it isn't one.
The usual caveat applies, mate: these matches are measured against the original, but finish and brand formulation change how a colour reads. Mylands has a particular depth and chalkiness in its Marble Matt that a standard Dulux vinyl matt won't fully replicate. If that richness is what drew you to Empire Grey in the first place, the difference will be in the *texture* as much as the hue.
Practical advice: order a sample pot of Bleached Lichen 1, brush out two coats on lining paper, and tape it up next to a Mylands sample card on the actual wall. Look at it morning and evening before you commit a full room.