Dulux Desert Vista is your match, mate. At a ΔE of 0.9 from Little Greene China Clay, the difference is imperceptible to the naked eye — anything under 1 means you'd struggle to tell them apart even with both samples side by side. It carries an LRV of 85.6, so it's a soft, gentle off-white that bounces light beautifully, much like the original.
If Desert Vista isn't easy to get hold of, Dulux Subtle Ivory 2 is right behind it at ΔE 1.2 and LRV 81.6 — still very close (anything under 2.5 is considered a very close match). It reads a touch warmer and slightly deeper, which can be no bad thing in a north-facing room where China Clay's coolness can tip a little flat.
Here's the honest bit, though: the reason people fall for China Clay in the first place is the depth of pigment and the chalky, soft finish Little Greene gets with their Intelligent Matt and Absolute Matt. A close colour match on a chart doesn't always translate to the same *presence* on the wall. Dulux's matt emulsion is a perfectly good, hard-wearing trade-friendly paint, but it won't have quite the same velvety quality at close range.
So if you're match-driven — repairing a wall, matching an existing scheme, or working to a tighter budget — Desert Vista will do the job and nobody will know the difference. If you're choosing China Clay for its character, I'd spend the extra and buy the real thing.
Whatever you land on, buy a sample pot and paint a decent-sized patch — A4 minimum, two coats — and look at it morning and evening before you commit. Charts and ΔE figures get you 95% of the way; your own light does the rest.