Mylands is gorgeous paint — properly pigment-rich, lovely depth — but you pay for it, and Long Acre No.102 is one of those deep, characterful greys 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. That's very close — you'd struggle to tell them apart on a wall, especially once the light shifts through the day. 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 very slightly lighter and a touch cleaner, but it's well within the range where most people would call it the same colour. 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 — 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 — and at ΔE 2.7 it should — you've saved a good chunk with nobody any the wiser.