Mylands is gorgeous paint — properly pigment-rich, lovely depth — but you pay for it, and Long Acre No.102 is one of those characterful mid greys (LRV 33.3) that people fall for and then balk at the price tag. So yes, there's a sensible cheaper route.
The nearest match is Dulux Steel Symphony 3 (LRV 33) at ΔE 2.7 from the original. Honesty first on that figure: it sits just past the very-close line, so hold the two side by side in good light and you'd pick them apart. On its own wall, in a real room, it reads the part — and once the light shifts through the day nobody's clocking it. Dulux costs significantly less per litre than Mylands, and if you go for Diamond Matt you get a wipeable, hard-wearing finish that's brilliant for hallways and family rooms.
Next best is Crown Curiosity (LRV 37.2) at ΔE 3.1. It reads slightly lighter and a touch cleaner — a visible difference if you put them next to each other, so treat it as a near neighbour rather than a twin. Crown is another budget-friendly option with a solid trade following.
I'd steer you away from COAT Renew for this particular match — it's a lovely paint and a more sustainable choice, but at ΔE 6.7 and LRV 27.3 it lands noticeably darker and won't read as Long Acre. If you specifically want COAT for the eco credentials, fine, but don't buy it expecting a dupe.
Here's the honest caveat, mate: Mylands' depth comes partly from how it handles light and from the quality of the finish itself, not just the hue. A ΔE 2.7 match gives you the colour near enough — it doesn't perfectly replicate that slightly velvety Mylands sheen. For most rooms that's a non-issue and the saving is well worth it.
Practical advice: buy a tester of Steel Symphony 3, paint two coats on a bit of lining paper, and tape it next to a Long Acre sample card in the actual room. View it morning and evening before you commit. If it holds up, you've saved a good chunk with nobody any the wiser.