LONDON — In the months before his attackers tracked him down, the exiled Iranian journalist had been moved in and out of safe houses by London’s Metropolitan Police, given a secret way to signal rescue units and had monitoring devices installed in his home.

       British authorities had done even more to protect Iran International, the London-based satellite news channel that airs the weekly program of the journalist, Pouria Zeraati, and has built an audience of millions in Iran despite being outlawed by the Islamic republic.