Honestly? There isn't a bang-on Dulux match for Edward Bulmer's Ethereal Blue — but Highland Falls 3 gets you the nearest at a measured ΔE 2.6, which is what I'd reach for if you want to stay in the Dulux range.
A word on the numbers. Anything under ΔE 1 is imperceptible to the eye, and under 2.5 is what I'd call very close. Highland Falls 3 at 2.6 is just over that line — so it's a strong match but a careful eye in good daylight might clock a slight difference, particularly in the green-blue balance. It carries an LRV of 45.5, so it sits mid-range and won't read as airy as some softer blues.
The other contender, Fresh Foliage (LRV 37.7), comes in at ΔE 3.5. That's a noticeable step further away — it reads greener and darker, so I'd only use it if you actually want more depth than Ethereal Blue gives you. It's not really a substitute for the same colour.
Now, the honest bit: Ethereal Blue is an Edward Bulmer colour, and a big part of what you're paying for there is the natural pigment and the chalky, light-handling quality that comes with it. A Dulux match will land the hue but it won't behave identically on the wall — Edward Bulmer colours shift beautifully through the day in a way mass-market emulsions don't quite replicate.
So my practical advice: if budget is driving the swap, go with Highland Falls 3 and order a tester. Paint a decent-sized board — at least A2 — and move it round the room across morning and evening light before you commit. North-facing rooms especially will pull any blue cooler, so check it there if that's where it's going.
If the room genuinely matters and you loved Ethereal Blue specifically, I'd spend the extra and buy the original. The match is good, not perfect.