You've got two perfect matches here, mate. Both Dulux Ashen White and Dulux Nougat Slice come in at ΔE 0 against Dulux Heritage Romney Wool — that's a dead-on match, completely imperceptible to the eye. Both also share an LRV of 71.6, the same soft, light reflectance you'd get from the Heritage original.
So why two matches for one colour? Dulux's standard mixing system carries thousands of shades, and it's not unusual for two named colours to land on the same point in colour space. For your purposes it makes no difference which you choose — ask for either at the counter and you'll get the same finish on the wall.
The reason people make this swap is straightforward: Dulux Heritage is the premium tier, with a flatter, chalkier finish and a higher price. The standard Dulux range is more widely stocked, cheaper, and available in trade-friendly options like Diamond Matt or Vinyl Matt if you want something more scrubbable for a hallway or kid's room. If you loved the Heritage *finish* specifically — that velvety, low-sheen Heritage Matt — then bear in mind a standard Dulux line won't replicate the texture exactly, only the colour. The shade will match; the surface feel can differ slightly.
Romney Wool sits in that warm, greige off-white territory — gentle, easy to live with, and forgiving across most light. At LRV 71.6 it bounces plenty of light without going stark, which makes it a sound choice for north-facing rooms where you want warmth without dinginess.
Practical tip: whichever you pick, buy a tester and paint two coats on lining paper, then move it round the room across the day. Even a ΔE 0 match can read differently depending on your existing trim and flooring. And if you're matching to an already-painted Heritage wall, take a sample chip in — the standard Dulux counter can scan it to confirm before you commit to a full tin.