I don't think John Harrison is someone from the Roddenberry universe. If you look at who he had target? He'd someone set off a exploded device in London England. He goes after star fleet, especially Pike and Marcus. Both of them had serve on the same ship sometime in the past together.
Maybe he's someone from pop culture, from the public domain. Tough British guy, who's available? Beowulf? Captain Hook? Allan Quatermain? Tarzan? Sherlock Holmes?