It's a long story, but here's the gist. Captain Britain was lost in the time stream while Rachel was still with Excalibur. Rachel was able to trade places with him (that is, he returned to the present while she became lost in time). It was later revealed that Rachel had wound up two millennia in a dystopian future ruled by Apocalypse. She basically founded the Clan Askani to give people hope and fight back against Apocalypse's oppressive tyranny. (Askani, which means "family of outsiders," was the closest she could think of to X-Men.)
Anyway, in the comics, it was the Clan Askani (not Bishop) who arranged for Nathan Summers to be transported into the future, where he had a chance of surviving the techno-organic virus that Apocalypse infected him with. Fortunately, they were able to save Nathan, but they couldn't cure him of the virus. So (1) they created a healthy clone of him, who grew up to become Cable's archnemesis Stryfe, and (2) they taught Nathan how to use his vast telekinetic powers to keep the virus from spreading throughout his body. FYI, Cable expends nearly all of his power to stave off the virus on a regular basis. If he wasn't forced to use his powers in this manner, he'd be able to unleash his full potential and become one of the most powerful mutants in existence.
I'm telling you, man. You've got to get your hands on The Adventures of Cyclops and Phoenix! It's a really great story by Scott Lobdell that features some of Gene Ha's best artwork to date.