It's probably more accurate to think of insider trading as being about trading on privileged access to information. Being non-public is more about who could, in principle, get that information rather than it being generally available.

In the Brexit case, there couldn't be an insider issue as the polling companies aren't inside anything relevant.

The grey area here is solely due to the law that says that exit polls can't be made public before the polls close. That is separate from insider laws but does relate to a question about what does "public" mean in that law.

In principle you might fall foul of insider trading if you had privileged information from the BofE, say, that they were going to make an unexpected rate move before it was announced. Similarly, if someone on Theresa May's staff bet against the pound on Sunday night, there may be a case as killing the vote on Monday was sure to trash Sterling.