Your best bet in the Dulux range is Neptune Seas (LRV 17.1), which lands at ΔE 2.7 from Little Greene Pleat. That's into very-close territory — under 2.5 is where you'd struggle to tell two colours apart side by side, so 2.7 means there's a whisper of difference but nothing most people would clock once it's on the wall.
The other contender is Midnight Garden (LRV 21), but at ΔE 4.2 it's a genuine step away — lighter and reading differently enough that I wouldn't call it a match. If you're after the same mood, Midnight Garden sits in the right family, but it isn't a substitute for Pleat. Go with Neptune Seas.
Now the honest bit. Why match at all? Usually it's cost, or a decorator who's set up with Dulux and doesn't want to source Little Greene. Both are fair reasons. But understand what you're trading: a 2.7 ΔE shift plus the difference in finish and pigment behaviour between the two brands. Little Greene's deeper colours have a particular depth and a slightly chalky, light-absorbing quality that Dulux's standard emulsions don't quite replicate. On a rich mid-tone like Pleat, that character is half the appeal.
If this is for a feature wall or anywhere the colour is doing real work in the room, I'd buy the actual Pleat. If it's a utility space, a rental, or you're tin-blind on budget, Neptune Seas will do the job nicely.
Whatever you choose — test it in the actual room first. A 2.7 ΔE difference can look like nothing in the tin and then read warmer or cooler depending on your light. Get sample pots of both, paint two coats on lining paper or a board, and move it around the room across the day. North light especially will pull these deeper tones cooler. Decide on the wall, not on the chart.