If you love Sanderson Newby Green but not the price tag, you're in luck — there's a genuinely excellent match sitting in the Dulux range.
Dulux Breton Blue (LRV 8.4) comes in at ΔE 1.3 from the original. Anything under 1.5 is effectively imperceptible to the eye in normal lighting, so once it's on the wall and dry you'd struggle to tell them apart. That's the one I'd reach for if saving money is the priority — Dulux is widely stocked, easy to get colour-matched, and the trade prices are a fraction of a heritage brand like Sanderson.
If you'd rather go modern-premium than budget, COAT's The Establishment (LRV 8) is the next closest at ΔE 2.6 — under 2.5 is what I'd call very close, and this is just a whisker over, so still a very respectable match. COAT's a cracking spec too: low-VOC, lovely flat finish, and they sell peel-and-stick samples so you can test before committing.
Crown Printworks (LRV 9.6) is the loosest of the three at ΔE 3.8 — you'd start to notice a difference side by side, so I'd only go there if you're already buying Crown for another reason.
A word of caution on these deep blue-greens: they shift hard with light. All three are sitting around LRV 8-10, which is properly dark, so they'll read almost black in a north-facing room and come alive in good daylight. Whichever you choose, buy a sample pot and paint a decent-sized patch — at least A2 — and look at it morning and evening before you order the lot.
My honest steer? Go Breton Blue. The match is close enough that you're effectively buying Newby Green for half the money, and that's a win in anyone's book.