OK. This is a strange one. To delete your WhatsApp account seems straight forward:
Go to Settings > Account > Delete My Account.
Enter your full phone number and tap Delete My Account.
That's it. This should be the point where you get a hug — but. WhatsApp have a strange message on their support pages that says Note: We cannot delete your account for you at this time 🤔
Why can't they actually delete your account? This is odd. We're trying to find out more. Will update soon.