The only way to cancel immediately is to use the PAC but you still get billed for the notice period even if you don't use it.
To get full value you need to use the PAC around 5 days before the end of your notice.
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.
Please select the post that helped you best and mark as the solution. This helps other members in resolving their issues faster. Thank you.