Strong White is one of those F&B colours people love but baulk at paying premium prices for, especially when they need a lot of it. Good news: you've got proper alternatives, and they're genuinely close rather than "close enough".
The standout is Crown Soaring Dove. At ΔE 0.2 from Strong White it's effectively the same colour — anything under ΔE 1 is imperceptible to the human eye, so you'd never tell them apart on a wall. LRV is 75.3 against Strong White's similar level, so it carries the same soft, slightly cool-grey character. Crown's Trade range gives you that match at a fraction of the cost, and the coverage is solid.
Dulux Love Letter Cf19 (LRV 75.5) is next at ΔE 0.5 — still well inside imperceptible territory. Dulux mix it to order, so availability is everywhere, and if you want their Diamond Matt for a more wipeable finish you've got that option too.
If you'd rather stay in the premium-feel camp but still save, COAT Sweatpants lands at ΔE 1 (LRV 76.5). It reads marginally lighter and brighter, but on a real wall you're looking at a whisker of difference. COAT's paint is low-VOC, washable, and the finish quality is genuinely lovely — a sensible shout if you care about eco credentials.
The "but what about" here: people worry a dupe won't behave the same in changing light. Strong White's appeal is how it shifts cool-grey in north light and softens in the sun. Soaring Dove and Love Letter both share that LRV band, so they'll do the same dance. The match isn't just the swatch — it's the behaviour, and these hold up.
My advice: get a tester pot of Soaring Dove and Sweatpants, paint two coats on lining paper, and move it round the room over a day. Buy the one that sits right in your actual light. At ΔE 0.2, Soaring Dove is the one I'd back.