Stonington Gray is a lovely balanced grey, but Benjamin Moore commands a premium price in the UK and the tins aren't always easy to get hold of. The good news is you can match it almost exactly for less.
The standout here is Crown Light Grey (LRV 52.6) at ΔE 0.7 from the original. That's well under 1, which means the difference is imperceptible to the human eye — put them side by side on a wall and you genuinely couldn't tell which is which. Crown is widely stocked, sensibly priced and their trade emulsion lays off beautifully, so this is the one I'd reach for.
If you'd rather a Dulux tin, Slow Living (LRV 52.6) comes in at ΔE 2.1 — still very close; most people wouldn't pick it out without the two brush-outs side by side. You'll find it easily, and the standard Diamond Matt or trade ranges will do the job nicely.
There's also COAT Sunday Soul (LRV 55.3) at ΔE 3.3. That's a touch further off — close, but the sort of gap you'd spot with the two side by side — though if you're already drawn to COAT for their low-VOC formulation and the convenience of ordering peel-and-stick samples, it's in the same family and worth a look. Just know it leans a touch warmer in undertone than the original.
The usual caveat applies: grey is a chameleon and any of these will shift with your light. In a north-facing room they'll lean cooler and can pick up a blue-grey edge; in warm afternoon light they'll soften. So whichever you pick, get a sample pot up on the wall — paint two coats on lining paper or board and move it around the room across the day before you commit. A ΔE of 0.7 on a chart means nothing if you've not seen it in your own light.
My money's on Crown Light Grey — it's the closest match and you'll save a packet.