Stiffkey Blue is one of F&B's best — that deep, slightly muddy navy with a grey undertow that goes almost black in low light and softens to denim where the sun hits. If you want the look for less, here's where I'd point you.
Dulux Midwinter Tide is your best bet. At ΔE 2.3 from Stiffkey Blue (LRV 10.1), it's genuinely close — under 2.5 means most people won't clock the difference once it's on the wall. You get Dulux's wider sheen range and easier availability, and you'll save a fair bit per tin. For a big feature wall or a whole room, that's the sensible call.
If you're after a real heritage-brand finish but a notch below F&B money, Crown Printworks (LRV 9.6) comes in at ΔE 3.8. Not as tight a match — it reads marginally different side by side — but on its own in a room you'd never know, and Crown's emulsion is solid for the price.
COAT The Establishment (LRV 8) sits at ΔE 4.3, so it's the furthest off of the three and runs a touch darker. Worth a mention because COAT's water-based formula is low-VOC and the colour itself is lovely — just don't expect it to be a carbon copy.
The honest caveat: at LRV around 8–10, all of these are properly dark. That means undertones get exaggerated by your light. North-facing rooms will pull them colder and gloomier; warm artificial light will lift them. Buy a sample pot of Midwinter Tide and paint it next to a Stiffkey Blue sample, both with two coats, and live with them for a couple of days before you commit.
One more thing — deep colours like this eat coverage. Use a grey-tinted undercoat (most of these brands offer one, or get your decorant's merchant to tint a basecoat) and you'll save yourself a third coat. That's where the real money saving lands.