If you want Skimming Stone without the Farrow & Ball price tag, Dulux Khaki Mists 5 is your colour. It comes in at ΔE 0.7 from the original — that's well under the threshold of 1, which means the difference is essentially invisible to the naked eye on a wall. With an LRV of 68.4 it carries the same soft, light, warm-grey quality that makes Skimming Stone so popular, sitting nicely in that off-white-leaning-grey territory.
Close behind is Dulux Weathered Pebble at ΔE 0.9 and an LRV of 67.7. Still very close, still a cracking match — the choice between the two often comes down to which one your local stockist can mix freshest or which mixing base you can get in the finish you want.
Now, the honest caveat: Skimming Stone is one of those F&B colours that behaves differently depending on the light. It's a warm grey with a faint mushroom undertone, and in a north-facing room it can read cooler and slightly mauve, while in good south light it warms right up. A Dulux match nails the colour at the mixing stage, but it won't perfectly replicate Farrow & Ball's pigment depth or the chalky way their estate finishes handle light. For most people, in most rooms, you genuinely won't clock the difference — but if you're a stickler who notices undertone shifts, that's the trade-off.
Practical advice: don't commit off a fan deck. Get a tester of Khaki Mists 5 in the actual Dulux finish you're using — Diamond Matt for high-traffic areas, vinyl matt for general walls — and paint two coats on a bit of lining paper. Move it round the room across a full day. If you're matching to existing Skimming Stone elsewhere in the house, do the test board right next to it. Get that right and you'll have saved a fair bit of money with nobody any the wiser.