Because of the length and the age of this thread, I’ll longer get notifications that you are waiting. Hence, the delay in getting back to you.
It depends why the brother is using the ID. It is probably a police matter. If the police aren’t interested, then you have no legal interest or standing in the matter.
If the brother is the executor of your best friends estate, he is entitled to see the emails.
An injunction application court fee is £255. Solicitors, if you are not doing it yourself, would cost probably another thousand pounds on top of that. If he defends the application, the court costs if you lose would probably be £5000 and possibly more.
If he is pressing you, pressing you, pressing you, pressing you, to do something, that is harassment and you are entitled to protection from harassment under the Protection From Harassment Act. That is a police matter. He either has to issue legal proceedings against you for whatever reason or leave you alone.
I can’t see the relevance of the essay that you were quoting about authority. I just don’t know where that fits into the equation.