Fugee

You sign a declaration stating that you will renounce your nationality.

From the IND website:
"Revoking and loss of nationality
Have you been convicted of a serious crime? Or have you committed fraud in obtaining a residence permit or Dutch nationality? This has serious consequences. Your Dutch citizenship can be revoked. This is even possible up to 12 years after you have obtained the Dutch nationality."

Reneging on your obligation in gaining Dutch nationality is fraud, right? I can well imagine the consequences are serious. I don't think, for example, a bank would take too kindly to somebody who fraudulently uses a Dutch passport to obtain a mortgage.

BritInParis

I don’t think that necessarily follows. You are a Dutch citizen until the Dutch government says you’re not. A Dutch passport issued whilst you’re a Dutch citizen is not obtained fraudulently.

Fugee

You're expected to send them proof. You get confirmation from the British authorities that you have renounced your nationality and you send that.

Here: https://ind.nl/Paginas/Afstand-nationaliteit.aspx - in Dutch