Mallow White is a lovely soft off-white with a gentle creamy warmth, but the Dulux Heritage range carries a premium over Dulux's standard offering. The good news is you don't need to pay it.
The standout here is Dulux Nordic Bliss, which comes out at ΔE 0 from Mallow White — that's a dead-on match, imperceptible to the eye. It sits in the regular Dulux line-up (LRV 80.9), so you get the exact same colour for noticeably less money. If you want the Mallow White look without the Heritage price tag, this is the one. No-brainer, frankly.
If you'd rather shop around, Crown Fresh Coconut is very close at ΔE 1.3 with an LRV of 83.7, so it'll read a touch brighter and cleaner — worth knowing if your room is north-facing and you want to claw back a little light. And COAT's Just, Barely lands at ΔE 0.7 (LRV 79.2), which is essentially identical and comes in COAT's low-VOC, fast-drying formula if that matters to you.
The usual "but what about" here: people worry a cheaper paint means a worse finish. It doesn't, necessarily — but be sensible about which Dulux product you buy. For walls, Dulux Diamond Matt gives you a wipeable, durable finish that's a genuine step up from the bog-standard matt and still cheaper than Heritage. The colour match is the colour match; the finish is a separate decision.
One practical tip: always test the dupe alongside the original on the actual wall before committing. ΔE 0 on paper is reassuring, but sheen level and your lighting will affect how it lands. Get a tester of Nordic Bliss, paint a big patch, and live with it for a couple of days. If it sits right — and at ΔE 0 it will — you've sorted the same colour for less.