Don't forget, if you are employed here you can pay voluntary class 2 in the UK, so about £200 per missing year.
If you are/were unemployed, you can only pay class 3, @£800


The govt website only shows you the missing years at class3.
If you want to pay class2, you call them, they send a bill, you pay it, then start with the DD if that's what you want.


Paying up the UK NI is a bargain, even at the class 3.