I'm not sure that some of the above is right. The way I think it works is:
If you are working, you can't avoid paying NI.
If you are not working, you will get credited with NI qualification years between age 60 and 65 - to count towards your basic State Pension 30 qualifying years (I appreciate this doen't help you).
If you are in a Contracted-out company pension scheme, you aren't paying into the state scheme. However you also aren't adding anything to your state entitlement (which will still exist for those years when you weren't in the company scheme).
If you are either in a contracted-in or a private pension scheme, you can't avoiding paying the reduced NI bit - even though it isn't helping your basic State benefit entitlement.
Swings and roundabouts, I'm afraid.