Classic Gray is one of Benjamin Moore's most-loved warm off-whites — a soft, barely-there greige that leans warm without going beige. Lovely colour, but BM isn't cheap and the stockist network in the UK is patchy. Good news: you've got proper alternatives that hold up under scrutiny.
The closest match is COAT Centred, measured at ΔE 1 from the original with an LRV of 74.8. Anything under ΔE 2.5 is very close; under 1 is essentially imperceptible to the eye, so Centred is the one I'd reach for if you want the colour spot-on. COAT is a cracking eco-credentialled paint, low-VOC, and their Soft Sheen and matt finishes are genuinely good.
If it's purely about saving money, Dulux Vintage Chandelier is your best bet — ΔE 1.1, LRV 75.6, and you can get it mixed at any merchant or DIY shed for a fraction of BM's price. For trade jobs where budget rules, this is the sensible call. Crown Chalk Hill sits just behind at ΔE 1.2, LRV 75.5 — also worth a look if your local stockist carries Crown.
Now the "but what about" — these are all in the LRV 74–76 range, which is properly light. In a north-facing room that warmth will read cooler and a touch greyer than it does on the chart, so don't expect it to glow. South-facing, it'll feel soft and creamy. That behaviour is true of Classic Gray itself, so the match holds regardless of orientation.
Whatever you choose, get a sample pot and paint two coats on a bit of lining paper, then move it round the room across the day. ΔE figures tell you the colour is right; they don't tell you how your light handles it. Tape it next to skirting and check it morning and evening before you commit.