Stone I is one of Paint & Paper Library's lovely soft off-whites, but you're paying premium money for it. The good news: there's a near-perfect dupe for considerably less.
Dulux Fine Cream is your answer. At ΔE 0.7 from the original it's an imperceptible difference — under ΔE 1 means your eye genuinely can't separate the two on a wall. It sits at LRV 88.8, so it's a properly light, bouncy off-white that'll lift a room without going stark. For most people redecorating a hallway, landing or living room, this is the sensible swap. Dulux is widely stocked, mixed in trade centres everywhere, and a fraction of the P&PL price.
If you'd rather stay clear of the big-box brands, Crown Collector's White is the next best thing at ΔE 1 (LRV 86.9). Still very close, still cheaper than P&PL, and Crown's matt emulsion has a nice flat finish that suits these chalky whites.
I'd steer you away from COAT No Offence as a like-for-like here — it lands at ΔE 2.8, which is past the "very close" threshold. It's a cracking paint (low-VOC, lovely application) but No Offence reads brighter and cleaner — its LRV is 99.3, near pure white — so it'll lose the soft, slightly warm character that makes Stone I worth having in the first place.
The usual caveat: the dupe matches the colour, not the finish or feel. P&PL's emulsion has a particular depth and a low-sheen quality that the data can't capture. If the room is a showpiece and the finish matters as much as the hue, get a tester of Fine Cream and the real thing side by side before you commit.
For everywhere else? Buy the Fine Cream, pocket the difference, and nobody will ever know.