You just use the global Poste Restante system. Give the vendor your name followed by 'Poste Restante' and the address of the Post Office nearest to you in the scheme from the website below. Its been around since God was a boy. You're welcome:
https://www.postoffice.co.uk/mail/poste-restante
https://www.postoffice.co.uk/mail/poste-restante