Nah. I think they precisely expected the English to send Alucard. The whole speech Rip received kind of suggested she was to be sacrificed for the greater good. Yet, someone as powerful as her needed to be sacrificed for Alucard to appear at all. I think the alarm signaled the time when the zeppelins approached London. Consequently it was also the time when Alucard needed to be away, so capturing the carrier had to happen well before that but not too early.