Your best bet from the standard Dulux range is Woodland Pearl 3 (LRV 42.2), which sits at ΔE 3 from Dulux Heritage Sage Green. The runner-up is Tuscan Glade 3 (LRV 45.6) at ΔE 3.8.
Now, a bit of honesty here. Anything under ΔE 2.5 is what I'd call very close — close enough that most people wouldn't clock the difference on a wall. Both of these matches sit just over that line, so they're in the territory of "clearly the same family, but a trained eye in the right light will spot it." Woodland Pearl 3 is the one I'd reach for first, but don't treat it as a like-for-like substitute if you're patching into existing Heritage Sage Green — you'll see the join.
Why the gap? Dulux Heritage is formulated as a separate, more pigment-rich range than standard Dulux, with deeper, slightly chalkier finishes. The standard line wasn't built to mirror it, so a perfect mathematical match doesn't always exist — and that's exactly the case here.
The likely "but what about" question: why not just buy the Heritage colour? If budget or availability is the issue, the standard Dulux match makes sense. But if you specifically want that Sage Green look and the wall is going to be a feature, I'd spend the extra and get the real thing — the depth is part of why you liked it.
Worth knowing too: Woodland Pearl 3 and Tuscan Glade 3 are both "3" steps within Dulux's tonal ladders, meaning lighter and darker versions exist on the same card. If neither lands quite right against your room, step up or down the ladder before abandoning ship.
Practical advice — always sample before committing. Sage greens shift dramatically with light. Paint a decent patch (A4 minimum) on two walls, live with it across a full day, and judge it in your own light rather than off a chip. A ΔE 3 match can read closer or further apart depending entirely on how your room handles north or south light.