Sanderson Eucalyptus is that lovely soft sage-grey that sits perfectly in a calm bedroom or a north-facing sitting room. If you want the look without the premium price tag, you've got two strong options.
Dulux Sea Urchin 2 (LRV 26.2) lands at ΔE 3.7 from the original. Crown Ivy Grey (LRV 26.7) is also ΔE 3.7. Both sit in that "close, but not imperceptible" bracket — a ΔE under 2.5 is what we'd call a near-perfect match, so at 3.7 you'll get a colour that reads the same on the wall but might show a whisper of difference if you held a brush-out card right next to the genuine Sanderson swatch. In practice, on a full wall and in normal light, nobody will ever know.
Of the two, I'd lean Crown Ivy Grey — Crown's matt has a nice chalky finish that suits this kind of muted green, and the LRV is almost bang on Eucalyptus's character. Dulux Sea Urchin 2 is the safer bet if you want it mixed at any trade counter with no fuss and you're matching it into an existing Dulux scheme.
Worth flagging: I'd steer you away from COAT Mr. Clifton here. It comes up as a candidate but it's ΔE 9.2 from Eucalyptus and a good bit darker at LRV 23.5 — that's a noticeably different, moodier colour, not a dupe. COAT make cracking paint, but it's the wrong horse for this particular race.
My advice: don't buy on the swatch card alone. Order a sample pot of whichever you fancy, paint two coats on a bit of lining paper, and prop it against your skirting in both morning and evening light. Sage-greys shift more than most colours depending on what daylight you've got. Get that right and you've saved a tidy sum without losing the finish.