Can a UK restaurant send WhatsApp offers to customers under PECR?

Yes, with consent. UK rules (PECR) need clear consent, or the soft opt-in for past customers only, and an easy way to opt out in every message.

• Consent: the ICO says you must not send marketing electronic messages unless the person specifically consented. A tick box can do it.
• Soft opt-in: covers existing customers who bought something similar, if you offered a simple opt-out when you took their details and in every message.
• No bought lists, and do not hide who you are.
• Meta also requires opt-in before you message people on WhatsApp.
• The ICO says its guidance is under review. Check the page. This is not legal advice.