Go for Dulux Moon Shimmer. It sits at a ΔE of just 0.5 from Edward Bulmer Spanish White, which is well under the threshold where the human eye can spot a difference at all (anything below ΔE 1 is effectively imperceptible). It's about as honest a match as you'll get across two different brands, mate.
Moon Shimmer carries an LRV of 92.3 — a soft, barely-there off-white that bounces plenty of light around without tipping into clinical brilliant-white territory. That's the appeal of Spanish White in the first place: a warm, chalky white with just enough body to feel calm rather than stark. Moon Shimmer holds onto that character.
If for some reason Moon Shimmer doesn't sit right under your particular light, Dulux Frosted Dawn is the next best thing at ΔE 1.6 (LRV 91.9). Still very close — under 2.5 — though you may notice the faintest shift in person. I'd start with Moon Shimmer and only fall back to Frosted Dawn if a tester says otherwise.
Now, the honest caveat: a colour match this tight gets you the hue, but not the pigment recipe. Edward Bulmer uses natural earth and mineral pigments, which is why their whites shift so beautifully through the day and never go flat. Dulux is a synthetic formulation, so under low evening light the two won't always behave identically — Spanish White will likely feel a touch warmer and more alive at dusk. For most rooms you'll never clock it, but if it's a feature space where the light really matters, that's the trade-off you're making for the lower price.
Practical advice: buy a Dulux Moon Shimmer tester, paint two coats on lining paper, and move it round the room across morning, midday and lamplight before you commit. And spec it in a matt or eggshell to match the soft, pigment-rich look you're after — a high sheen will undo the chalky calm that drew you to Spanish White in the first place.