why would they care about the story of some Jewish guy 1500 years ago?

Well...they didn't. The initial efforts at converting native Britons to Christianity happened under the Roman Empire, pre-400. The big successful evangelizing effort that we usually think about is Augustine of Canterbury's mission under Pope Gregory I, which started at the very end of the 6th century. A lot of credit for his success goes to the fact that he converted the king of the specific kingdom he was in (Kent) to Christianity first, which opened the door to the establishment of dioceses and more importantly monasteries. People aren't converting because of events "1500" years ago; they convert because of what they see and feel happening around them now.

And the 'historical' aspect is hardly a dealbreaker, in any case. I don't understand why you keep emphasizing "Jewish." Christianity doesn't--medieval Christianity really didn't. There wasn't a sizable presence of Jews in western Europe--let alone the far northwest--in the early Middle Ages. This difference wouldn't have meant anything to most English people in the early days even if Augustine et al had stressed it.

People like the past and draw meaning from it. There's also no obvious connection between medieval England and ancient Troy, yet Geoffrey of Monmouth's Historia Regum Britanniae--also known as the chronicle that more or less gives birth to the written Arthurian saga--roots the kingship of England in, you guessed it, ancient Troy.

What part of English history are you thinking of in particular?