Dulux Snow Scene is the one to reach for. At ΔE 0.6 from Mylands Pure White No.1, the difference is genuinely imperceptible — you'd never spot it on a wall, even side by side. With an LRV of 90.3 it's a bright, clean, high-reflectance white that does exactly what Pure White No.1 does: bounces light around without tipping cool and clinical.
If you want a sliver more warmth and depth, Dulux Cliff Walk is the runner-up at ΔE 0.8 (LRV 88.6). Still very close — under 2.5 is what we call a very tight match — and the marginally lower LRV means it reads a touch softer in a room that gets blasted with bright daylight. Both are well within the range where nobody but a spectrophotometer could tell them apart from the Mylands.
Now, the honest bit. Colour match doesn't mean *finish* match. Mylands paint has a particular depth and a beautifully flat, chalky finish in its Marble Matt — and a higher pigment load that Dulux's standard ranges won't quite replicate. So if you're matching for a touch-up on an existing Mylands wall, the colour will line up but the sheen and the way light catches it may not, especially in raking light across a feature wall. For a full repaint of a room, none of that matters — Snow Scene will give you the same look for less money.
My advice: order a Dulux sample pot of Snow Scene and brush a decent patch — at least A4 size, two coats — next to a Mylands sample if you've got one. Whites shift more than people expect with orientation and the light bulbs you've got in. North-facing rooms will pull any white slightly cooler, so judge it in the actual room at the times of day you use it most.