At the beginning and end of the day, it's your game dude. If you want to run Magical London as a city that has so much history it needs like 12 aspects to fit it all, go ahead. But, for every Theme and Threat you will a have 1 face that upholds that aspect and one that rocks the boat. You can do it, but you might need to do some more work than normal.
I would suggest you talk it over with your group and ask them what is important to them. Whatever Themes and Threats you pick, they will come up throughout your campaign.
In terms of Themes/Threats, just remember this rule of thumb: It is a
Theme if it is a normal part of everyone's day and it's existence doesn't really change things, it is a
Threat if it can and will ruin the lives of the City and nothing else.
Though you might make and entire campaign just going from theme to threat to theme. After all, you can only blow up Big Ben with Fire so many times....