I er, kind of don't want to write a SJA/YW crossover where Luke finds an advanced quantum physics textbook while in university which is actually a manual and becomes a wizard.
Not because it's unbelievable, but because there'd be too many OCs taking major roles like the district overseer person and someone who'd explain what this thing was.
And then there's the things like Mr. Smith being told and then going "Oh, that thing. I would rather not but I do have experience constructing spell strings" or meeting someone who asks if he's the son of the assumed overseer of the London area because when crises popped up requiring that kind of authority, Sarah Jane's name popped up. (Are aliens crash landing on Earth wizard-worthy? Especially when they intend to destroy/take over Earth? I mean lets just assume all peaceful aliens are actually on errantry and that the manuals also recognise that Sarah Jane can help with that too and list her name and location for off-worlders seeking help.) And of course the big, OF COURSE IT MAKES SENSE, is that Luke WOULD be able to string together a multi-clause spell with many variables in two minutes.