If you want Long Acre in a Dulux pot, Steel Symphony 3 is your best bet. At ΔE 2.7 from the original it's a very close match — close enough that most people wouldn't clock the difference on a wall, though a trained eye might spot a hair's difference side by side. Its LRV of 33 sits in that mid-depth, soft-grey territory Long Acre lives in, so it'll behave similarly in terms of how much light it bounces back.
The runner-up is Quartz Flint 1 (LRV 29.5), but at ΔE 4.6 it's a noticeably different beast — a touch darker and off in undertone. I wouldn't reach for it expecting a faithful copy. Use it only if you actively prefer how it looks, not as a Long Acre substitute.
Here's the honest caveat, mate: a ΔE of 2.7 means the match is good but not invisible. Mylands paints carry a depth and slight chalkiness in the finish that Dulux's formulation won't replicate exactly, even with a spot-on colour reading. So if you're patching into existing Long Acre, don't switch brands mid-wall — you'll see the join in certain light. Match the brand for repairs.
Where brand-swapping makes sense is a fresh start: a new room, a whole redecoration, or where budget's tight and you'd rather put the saving into a better roller and two proper coats. In that case Steel Symphony 3 will give you a thoroughly convincing Long Acre look for less outlay.
Whatever you decide, always test before committing. Get a sample of Steel Symphony 3, paint two coats on a bit of lining paper or board, and move it around the room over a full day — north light, afternoon sun, lamplight. Greys of this depth are the most prone to shifting undertone depending on aspect, so judge it in your actual space, not in the shop.