If you're after a Dulux stand-in for Little Greene Middle Buff, go with Dulux Sunflower Symphony 2. It comes in at ΔE 0.7 from the original, and anything under ΔE 1 is effectively imperceptible to the human eye. In practice that means once it's on the wall, dry and viewed in normal light, nobody will clock the difference. It carries an LRV of 22.1, so it sits in that warm, earthy mid-tone territory Middle Buff is loved for — a soft mustard-gold that reads cosy without going full ochre.
If for some reason you can't get Sunflower Symphony 2 mixed, your back-up is Dulux Tuscan Treasure 1 at ΔE 2.4 (LRV 23.3). That's still a very close match — under ΔE 2.5 is the threshold most decorators treat as "you'd only see it if you held the two side by side." It's a touch lighter and very marginally different in undertone, but on a full wall it'll do the job.
Now, the honest bit you should hear: a colour match gets you the hue, not the paint. Little Greene's finishes — particularly their Intelligent Matt and Absolute Matt — have a depth and chalky quality that Dulux's standard emulsion doesn't quite replicate. Middle Buff is a colour that genuinely benefits from that richness, because deeper earth tones can look flat in a cheaper-bodied paint. So if budget allows and the room matters, I'd stick with the real thing.
Where the Dulux match earns its keep is the practical stuff: a big job where you need litres of paint, trade availability, or matching an existing scheme without paying premium prices throughout. In that case, get Sunflower Symphony 2 mixed in Dulux Diamond Matt for durability or standard Vinyl Matt for a quieter sheen.
One tip — always brush out a tester and live with it for a couple of days. Buff tones shift noticeably between north and south light, and you want to see it on your wall before committing.