Purbeck Stone is one of F&B's most quietly useful greys — a warm, grounded mid-tone that behaves well in north and south light alike. The good news is you don't have to pay F&B money to get it.
The nearest match in the FiniSpec library is Crown Plymouth Grey at ΔE 2 (LRV 48.3). That's very close — well under the 2.5 threshold where most people stop being able to tell two colours apart side by side. Crown's trade emulsion is a fraction of the F&B price, covers properly, and you'll struggle to spot the difference once it's on the wall and dry.
Right behind it is Dulux Slow Living at ΔE 2.3 (LRV 52.6). It sits slightly lighter and a touch airier than Plymouth Grey thanks to that higher LRV, so if you found Purbeck Stone a smidge heavy in your room, Slow Living nudges it brighter without losing the warmth. Dulux Heritage in this tonal family is also worth a look if you want a chalkier finish.
If you'd rather stay in the premium-but-not-F&B bracket, COAT Sunday Soul comes in at ΔE 2.9 (LRV 55.3). That's a bit further off — noticeably lighter and the warmest of the three — so think of it as Purbeck Stone's brighter cousin rather than a straight dupe. COAT's eco credentials and low-VOC formula are the draw there.
My pick: Crown Plymouth Grey for the tightest match and best value, or Slow Living if you want it a hair lighter.
One practical bit — colour matches are measured under standard light, but your room isn't a lab. Buy a tester of whichever you fancy, paint two coats on a bit of lining paper, and move it round the room across the day before committing. ΔE 2 is reassuring, but your own eyes in your own light are the final word.