Go for Dulux Chiffon White 4. At ΔE 0.4 from Little Greene Slaked Lime, that difference is genuinely imperceptible to the eye — you'd struggle to tell the two apart on a wall, let alone fault the match. With an LRV of 85.9 it's a soft, barely-there off-white with that same gentle warmth that makes Slaked Lime so easy to live with.
If Chiffon White 4 isn't available in the finish you need, Dulux Cloudy Dreams is the next best thing at ΔE 1 (LRV 84.2). Still very close — under 2.5 ΔE is what we'd call a confident match — just a touch deeper. Either will do the job.
Now, the honest bit. Matching the *colour* is the easy part. What you don't get when you switch from Little Greene to Dulux is the pigment depth and the way Little Greene's paints handle light through the day. Slaked Lime has a quiet, chalky quality that comes partly from the formulation, not just the hex value. On a north-facing wall in flat light you'll likely never notice the difference. In a bright south-facing room with strong raking light, the Little Greene version may hold its character a shade better. That's the trade-off you're paying for.
That said, for most rooms the saving is well worth it, and Dulux's coverage and durability are excellent — particularly if you're after their Diamond Matt for a hard-wearing finish in a hallway or kids' room.
My advice: buy a tester of Chiffon White 4, paint two coats on a bit of lining paper, and prop it against the wall at different times of day before you commit to a full tin. Match figures are a brilliant starting point, but your own room and light always get the final say.