Either call customer services who will post you one out with instructions on how to activate it
http://swapmysim.o2.co.uk/ http://www.o2.co.uk/contactus or pop into your local o2 shop where they will do it for free while you wait.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion.
This is not Customer Service so we are unable to help with account specific issues.Currently using:
Pixel 7a (O2 & Lyca), One Plus 6 (Sfr), iPhone 12 Pro Max (Vodafone)