As a Class 2 payer, my biggest fear is getting to pensionable age and then HMRC telling me : "Oops, sorry! You should have been paying Class 3 all these years....now you have a greatly reduced pension".I just spoke to HMRC now and got through to an agent who told me that "Class 2 is only for people who were self employed prior to moving abroad." That doesn't match the information that I have read online.
I wouldn't put it past them.
I wasn't self-employed before coming to Japan; I was just a regular employee, so no idea why they accepted me for Class 2....