Gray Owl is a cracking colour — a soft, cool grey with just enough green to keep it from going cold and clinical. But Benjamin Moore isn't the cheapest paint on the shelf, and stockists in the UK can be thin on the ground, so I don't blame you for hunting an alternative.
The standout is Dulux Spun Mohair (LRV 66.2). At ΔE 1.2 from Gray Owl, that's so close you'd struggle to tell them apart on a wall — well under the ΔE 2.5 threshold where differences become genuinely noticeable. It's a fraction of the price, widely stocked, and Dulux Diamond Matt gives you a tough, wipeable finish for hallways and kitchens. That's the one I'd reach for.
If you fancy something with a slightly higher light bounce, COAT Rathbone Place (LRV 69.5) matches at ΔE 1.4 — also imperceptibly close. COAT's eco credentials are solid, the finish is lovely and matt, and they deliver direct, which sidesteps the stockist hassle entirely. Worth a look if you like ordering online.
Crown Sugar Bowl (LRV 69.9) sits a touch further out at ΔE 2.2 — still a very close match, just reading marginally lighter and brighter. Perfectly usable, and Crown is keenly priced through trade counters.
The "but what about" here: Gray Owl is a notorious chameleon. It can read pale grey, soft green, or even faintly blue depending on your light. North-facing rooms pull it cooler and greener; south light warms it right up. Whichever of these you choose, the same applies — so don't commit off a chart.
Get a sample pot of Spun Mohair and one of Rathbone Place, paint two coats on a bit of lining paper, and tape it to the wall for a couple of days. Check it morning and evening before you buy the lot. Do that and you'll have Gray Owl's look for a good deal less, sorted.