If I were to build a step 15cm high leading onto a deck, how
big or little would that step have to be before it became a 'deck'?

That which a reasonable person would view it as a "deck".

It comes down to a test of reasonableness re planning officer.
Legislation context is w.r.t. "overlooking". Magistrates will not
entertain an idiot PP over a tiny step provided for implied H&S usage
of a deck.

Build your deck below 30cm.
Build any step sized only to provide safe entry/exit to deck.

I see no problem with an intermediate step sized to door-width & stair-
depth to adjust 45-35cm house-floor to 30-0cm deck. The deck is below
30cm. The step must not remotely be sized as a "deck" by any
reasonable person's interpretation. You can't take the ****, but
*they* can try - at their expense and media embarrasment and household
insurance lawyers. Full stop.

Which PP wants to be the first to put its hand up for cost cutting on
Newsnight?