If you want Cornforth White's look on a Dulux budget, Dulux Pale Taupe is your colour. It sits at LRV 61.4 with a measured ΔE of just 0.7 from the original — anything under 1 is imperceptible to the human eye, so on a wall you genuinely won't be able to tell them apart.
Your second option is Dulux Pebble Shore (LRV 62.4), which comes in at ΔE 1.5. Still a very close match — anything under 2.5 reads as the same colour in most lighting — but Pale Taupe is the one I'd reach for first.
Now, the thing about Cornforth White: it's a mid grey that plays games with light. In a north-facing room it can throw a cool, faintly mauve cast; in warm afternoon light it softens right out. The Dulux matches will do the same, because a good colour match isn't just hue — it's the undertone behaviour too, and both these Duluxes carry the same grey-with-a-warm-grounding character.
The "but what about" question I always get: *will it actually look the same once it's up?* The honest answer is the colour will, but the finish won't necessarily feel identical. Farrow & Ball's Estate Emulsion has a particular chalky, ultra-flat dead-matt quality that Dulux's standard matt doesn't quite replicate. If that velvety depth is what you're after, go for Dulux Heritage Velvet Matt rather than the trade matt — it gets you closer on texture as well as colour.
Practical advice: order a Dulux Pale Taupe tester and paint two coats on a bit of lining paper, then move it round the room at different times of day before you commit. A 0.7 ΔE match on a chart is one thing; how it lives with your light, your floor and your sofa is what actually matters. Do that and you'll have it sorted for a fraction of the F&B price.