Meta has multiple server relay nodes in EU. Just shutfown the relay nodes in UK and geolocation block all incoming connections to EU servers coming from UK IP address. The phone number is just at user registration. Back-end they know. Heck, my whatsapp even route my voice call info (likely they transcribe and send to FB adverts processing via chatGPT-like solution) and flooded my FB with related ads derived from my voice call keywords.