Pleat is one of Little Greene's lovely muddy teal-blues, and if you want that look for less, Dulux Neptune Seas is your best bet. It sits at ΔE 2.7 from the original (LRV 17.1) — close enough that you'd struggle to tell them apart on a wall once the light's doing its thing. For a near-imperceptible match you'd want ΔE under 2.5, so Neptune Seas is just shy of that, but it's the tightest match here by a clear margin.
If you'd rather a brand with a bit more depth and a premium-ish finish without the Little Greene price tag, COAT Adulting comes in at ΔE 3.3 (LRV 16.2). COAT's a proper modern outfit — low-VOC, decent coverage, and you can order peel-and-stick samples to check it in your own room before committing. That's worth doing with any teal, because they shift hard between north and south light.
The one I'd steer you away from is Crown Botany Bay — at ΔE 5.6 it's noticeably greener and lighter (LRV 22.4), so it reads as a different colour rather than a dupe. Fine on its own merits, but not a Pleat substitute.
Now, the honest bit: with deep, complex colours like Pleat, a low ΔE on paper doesn't always tell the whole story. Little Greene uses richer pigment loads, so Pleat can have a slightly different undertone shift as the light fades. If you're doing a feature wall or a small room where it'll be scrutinised, pay for a tester pot of both Neptune Seas and Pleat and paint A4 boards side by side. If it's a hallway, utility, or somewhere you're not living in the colour all day — Neptune Seas will save you a tenner-plus a tin and nobody will ever know.
Whatever you land on, two coats over a grey primer or a tinted basecoat for these darker teals. Skimp on coats and the depth goes flat.