It's like contribution to IRA in USA, you must have income to contribute. HMRC withheld just income tax on that amount we withdrew. But we paid NI separately for years to get UK SS pension.
I was talking about my UK private pension, I'm not old enough yet to start drawing my UK SS (aka OAP) to which I continue to pay voluntary NI contributions. If the taxation agreement states that the UK is the primary taxation authority then claiming the tax back using IRS form 1116 should work, and the creation of the substitute 1099-R should provide a documentation trail of the source of income. (I'm not certain where the U.K. OAP should be reported on the 1040 as I've never had to do it)