Dulux Valley Rock is your best bet, mate. At ΔE 1.6 from Little Greene's French Grey it's so close the difference is essentially imperceptible — you'd struggle to tell them apart on a wall, even side by side. It sits at LRV 54, so it carries the same soft, light feel as the original: that gentle greened-grey that goes warm in the sun and cooler under overcast skies.
If Valley Rock isn't to hand, Dulux Lunar Landing is the next nearest at ΔE 2.3, with an LRV of 52.7. Anything under 2.5 is still a very close match, so this one won't let you down either — it's a hair deeper and reads very slightly more neutral, but in most rooms you'd never clock the difference.
Here's the honest bit, though. The reason people fall for French Grey is the *quality* of Little Greene's pigment — that lovely depth and the way it shifts through the day. Dulux nails the hue, but the finish behaves differently. Little Greene's Intelligent Matt and Absolute Matt have a chalkier, flatter look that Dulux's standard matt doesn't quite replicate. So you're matching the colour, not the character.
My advice: if French Grey is going somewhere it'll really be seen — a hallway, a feature room with good light — pay for the real thing. If it's a back bedroom, a rental, or you're doing a big square-meterage job and the budget's tight, Valley Rock will do you proud and nobody will be any the wiser.
Whatever you pick, get a proper sample pot and paint a big swatch on two different walls before you commit. Greys this delicate are notorious for picking up whatever's bouncing around the room — green from the garden, warmth from a south-facing window. Live with it for a couple of days through changing light, then decide. That's the only way to be properly sorted.