Like others I list NS&I premium bonds as regular accounts on the FBAR. I do have an income bond with NS&I too, which I show as 'securities', as its a loan to the U.K Gov. Although I simply receive interest from that account. When I first opened, this is how I reported. For consistency, I continue to report this way and will do so up until the account reaches it's natural maturity and closes.
IMHO, Once you report in one maner; best to continue, It may raise questions otherwise. Unless it's obviously wrong, (you need to amend a FInCEN 114), i'd keep things as they are. Ultimately, you've reported the account and maximum value.