Salt V is a lovely soft cool grey, but Paint & Paper Library sits at the premium end, so I understand wanting to bring the cost down. The good news is there are genuinely close matches.
The best of the bunch is Crown::Light Grey at ΔE 1.8 from the original. Anything under 2.5 is what we'd call very close — on a wall, in normal light, you'd struggle to tell them apart. Crown's a fraction of the price and widely stocked, so it's my first call. It reads slightly lighter, with an LRV of 52.6, so it'll bounce a touch more light around the room — handy in anything north-facing or a bit dim.
If you'd rather stay with Dulux, Misty Mountain lands at ΔE 2.2 with an LRV of 45.3. That's still very close, and Dulux is everywhere — trade counters, sheds, the lot — so it's the convenient choice if you want to grab a tester this afternoon and crack on.
Worth a mention is COAT::The Trail at ΔE 2.9 (LRV 44.7). It's just outside that very-close band, so expect it to read a hair different side by side — but COAT is properly low-VOC, water-based and self-priming on sound surfaces, which matters if you're after a cleaner formulation or doing a kid's room.
The usual caveat applies, mate: a low ΔE means the colour is matched, but sheen and finish change how a colour reads. Salt V in a flat emulsion will look softer than the same colour in an eggshell, which throws more light. So match the sheen level too, not just the colour.
Whatever you land on, buy a tester and paint two coats on a bit of lining paper, then move it round the room across the day before committing. Grey is the trickiest family for shifting under different light — that's a half-hour that saves you a re-paint.