If you're after Dock Blue without paying Little Greene prices, Dulux Heritage Oxford Blue is your best bet. It lands at ΔE 2.5 from the original — that's within the "very close" bracket, where most people won't clock a difference once it's on the wall and dry. Oxford Blue sits at LRV 3.8, so it's properly deep and inky, much like Dock Blue itself.
The runner-up is Dulux Slow Swing at LRV 3.2, but at ΔE 3.7 it's a noticeably looser match. You'd see the shift if you held the two side by side — Slow Swing reads a touch differently in the undertone. I'd only reach for it if Oxford Blue isn't available to you for some reason.
A word on why these matches matter on a colour this dark: rich, saturated blues are unforgiving. Tiny differences in undertone get amplified in low-light conditions and under artificial lighting, where a green-leaning blue can suddenly read teal and a red-leaning one can go almost navy-purple. So even a ΔE of 2.5 is worth proving out before you commit a whole room.
The "but what about the finish?" question is the real one here. A match figure compares colour, not sheen or how the pigment behaves under a roller. Dulux Heritage in their Velvet Matt won't lay down identically to Little Greene's Intelligent Matt or Absolute Matt — the depth and the way light sits on the surface can differ even when the colour reads the same.
So do this: get a sample pot of Oxford Blue, paint two coats on a decent-sized piece of lining paper (A3 minimum), and move it round the room across a full day. Check it against north light and your evening lighting. With deep blues, that test is non-negotiable — get it right and Oxford Blue will do Dock Blue's job nicely.