Sanderson Scotch Grey is a lovely soft mid grey, but you don't need to pay Sanderson money to get it on your walls. There are two genuinely excellent matches.
Dulux Shingle Steps (LRV 58.9) comes in at ΔE 1.2 from the original — that's the threshold of what the human eye can detect, so once it's dry on a wall in real light you'd never tell them apart. It's the one I'd reach for first, partly because Dulux is everywhere and easy to get colour-matched into the trade emulsions if you want a tougher finish for a hallway or stairwell.
Crown Needles & Pins (LRV 54.6) is just as close at ΔE 1.2. It sits a touch deeper than Shingle Steps thanks to the slightly lower LRV, which can read as marginally more grounded in a north-facing room. Crown's Easyclean Matt is a cracking value option if you want a wipeable wall without paying premium prices.
The one to be careful with is COAT On Mute (LRV 66.8) at ΔE 3.9. That's a noticeable step away — it's lighter and reads softer, more of a chalky pale grey than a true match. If you specifically want the Scotch Grey look, On Mute won't get you there. But if you're flexible and just like the family, COAT's water-based paint is genuinely good and the colour is a nice one in its own right.
My honest advice: order sample pots of Shingle Steps and Needles & Pins, brush them out on lining paper next to your Sanderson swatch, and view them morning and evening. Grey shifts more than any other colour with the light — what looks identical at midday can diverge at dusk. Both are so close that the deciding factor will be which brand's finish and price suits your job. Either way, you'll save a fair bit over Sanderson without anyone being the wiser.