Dock Blue is one of Little Greene's deep, slightly green-leaning navies — gorgeous, but you're paying premium prices for it. The good news is there's a genuinely close alternative that'll save you a fair bit.
Dulux Heritage Oxford Blue is your best bet at ΔE 2.5 from the original (LRV 3.8). Anything under 2.5 counts as a very close match — the kind of difference you'd only ever spot with both samples side by side under controlled light, never once it's up on four walls. The DH line is properly pigmented and lays down beautifully, so you're not compromising on finish either, just on the badge.
If you fancy something from the colour-confident crowd, COAT David Rose comes in at ΔE 3.8 (LRV 4.1). That's a touch further off — you'd notice a slight shift if you held a Dock Blue card against it — but it's still in the same deep-navy family and COAT's water-based eggshell is a cracking trade-quality paint.
I'd steer you away from Crown Midnight Navy here. At ΔE 6.7 (LRV 6.6) it's a clearly different colour — lighter and reading more straight-blue than the green-tinged depth Dock Blue has. It's a lovely navy in its own right, just not a stand-in for this one.
One thing worth saying: deep colours like this are the hardest to match because the eye is brutal on dark shades — small differences in undertone show up more than they do on a pale neutral. So if you're matching to existing Dock Blue already on a wall, order a tester of the Oxford Blue and brush it out next to it before you commit to a full tin. Dark navies also need two solid coats over a grey primer to avoid patchiness, whichever brand you land on. Get a tinted undercoat down first and you'll save yourself a third coat.