This is cannon until S6. Any other changes are minor and don't really change the events. While this is a crossover no crossed over things will be mentioned in it. It is the cross over aspect that results in the changes.
Disclaimer: I do not own Buffy or Dr. Who. If someone wants to give them to me I will not stop them at all.
Magic always has a price equal to that which it provides. So it comes as no surprise that the creation for the slayer line came with a hefty price tag. The spell takes a child-bearer from them, but that could hardly be sufficient to pay for the slayer. And it was not until Giles was reading one of his families journals that he discovered the true depth of the price. For you see slayers, without exception, are the most intelligent beings on earth. His ancestor only found this out because his slayers entrusted this to him. Giles ancestor expounds upon this by explaining all the miraculous inventions the three slayers he cared for created to help with the slaying. Giles was astounded to realize that these 5th century girls created holy water grenades, a communication devise that he located that still worked, an alloy that was stronger than anything they had even today, and most surprising of all, a set of glasses that showed body heat. Glasses that he wore from then on out.
So, not only was a child-bearer selected but also on of the worlds brightest mind. But the council had no record of this . His ancestor explained that the slayers did not trust anyone with this secret the felt a need to hide their intelligence and that they did not trust the council in the least.
Giles will admit that when he first met Miss Summers he had thought his ancestor wrong. But then after awhile he saw very small traces of her being smarter than she appeared, but nothing to indicate that she was the most brilliant mind on earth. So, he dismissed his ancestor foolishly n favor of what his eyes told him. That was until the joining spell to defeat Adam. It was then that he caught a small glimpse of her true nature and he only caught that because he was the mind of the group. It was when he was reflecting on that experience that he realized he had been wrong in dismissing his ancestor. That led to him thinking what could he do to get closer to her to see the real her. But then Glory came and there was no time and then... then it was to late for anything. She was gone, forever.
He looked at her and knew..just knew without a doubt that she was lying. She did not have the look of a survivor of a horrible experience but one who was living one right now. It was then he decided he had to talk to her, and revel that he knew the truth of her inelegance. To reveal that he knew she could not have been on hell to reveal that he was there for her. He was resolved to do so. And then he did it.
Rupert Giles never realized that his resolve to do the right thing changed the course of the future events and opened the way for the greatest war to ever exist, anywhere, anytime, anyever. A battle for existence it self, a battle that would be fought, had been fought and was being fought right then and there. Rupert Giles through his love for his charge allowed Good to be in the fight for realty.