Using Periods for Abbreviations

Okay, this may sound totally inane, but I just realized that I'm using periods for some abbreviations and not others. I'm wondering if I need to be consistent, or can you determine on a case-by-case basis?

For instance, I refer to Britain as the "U.K.", and then in the following sentence, I refer to someone's "NI number". I like the idea of being consistent, but I don't want to get this wrong, either. Is there a rule? Specifically, is there a British rule, since these are both British examples?

Lyra said:

British usage doesn't normally bother with the full stops in such abbrevations nowadays - so it would be "UK" and "NI".

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Thanks for clarifying that!

But if you mean "Britain" you shouldn't be saying "UK" anyway - they're not exactly the same places.

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Is it not a UK passport, then? I'm aware of the differences between "England", "Great Britain", and "the UK", but I was under the impression that a passport was for the entire UK. As far as I know, I'm using the terms correctly within the story, but I did make an assumption about the passport, so please correct me if I'm wrong.

Lyra said:

It is a UK passport (says on it United Kingdom of Great Britain and Northern Ireland) and covers the whole UK, but it's always called a *British* passport.

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Interesting. Well, that's good to know. Thanks, I'll be sure and change that detail.