Sure, it’s easy to find the Doctor in WWII London by scanning for him, but he visits there so often there’s no guarantee that you’re going to find the right Doctor.
Likewise, if you’re scanning for alien tech during the Blitz, you’re going to turn up one or two surprises. First of all, you’re going to suddenly discover that the air everywhere is filled with nanogenes, thus solving the mystery of the gas mask people an episode and a half early, which is good news, because you don’t have to spend all that time hanging out with Rose and the Doctor, developing a moral compass and eventually getting killed and unkilled for the rest of your life! Pluswise, you’re also going to find daleks, Chronosteel, Spitfires with the potential for spaceflight and a robot who thinks he’s a man.
And finally, Jack will probably end up finding himself, since he strongly implies that the con he’s currently running frequently brings him to London at the height of the Blitz. Additionally, the year is 1941, so he and Tosh are also out there, somewhere, meeting the original Jack Harkness and hiding out in an air raid shelter with him. PLUS, the immortal version of Jack had to get back to the 21st century the long way ‘round, by living through everything post-1869, and if you’re telling me that a man hell-bent on meeting the Doctor isn’t going to turn up in WII London, where he knows for a fact the Doctor will eventually be, then I’m not sure we’re both ready to discuss this subject.
