Silent White is one of Little Greene's lovely soft off-whites, but if you're painting a whole house the premium adds up fast. The good news is there's a genuinely close match in the budget bracket.
Dulux Apple White is your best bet. At ΔE 0.7 from Silent White it's an imperceptible difference — anything under 1 means you'd never spot the gap on the wall, even side by side. With an LRV of 86.6 it bounces light around just like the original, so it'll behave the same way in a north-facing room or a bright south-facing one. For the money, it's a no-brainer.
If you'd rather stay with a trade-friendly brand, Crown Collector's White comes in at ΔE 1.2 (LRV 86.9) — still very close, and Crown's Trade Matt and Clean Extreme finishes are solid, hard-wearing options for hallways and kitchens.
The outlier is COAT Jane at ΔE 2.5. That's still within "very close" territory, but at LRV 95.2 it's noticeably brighter and whiter than Silent White — less of that soft, grounded warmth. It's a cracking paint (eco credentials, peel-and-stick samples), but it's not a true like-for-like swap. Choose it because you like Jane, not because you want Silent White on a budget.
One thing worth saying: a colour match gets you the colour, but not the texture or depth of pigment that Little Greene builds into its paints. On a feature wall in a fancy drawing room you might notice the difference in how light sits on the surface. On general walls, skirting and ceilings? You'll never tell.
My advice — get a tester of Apple White, paint it next to a Silent White sample on the actual wall, and look at it morning and evening. If you're happy (and at ΔE 0.7 you will be), pour the money you've saved into a better roller and a decent primer.