Infrastructure? Who needs it? What you need is a couple of catapults, a few horsemen and maybe one spearsman. Kill the enemies with your horses, protect the catapults with the spearsman, siege London and in 10 turns you should have one extra good city and some trained units. Unless you are playing Deity or something like that.
EDIT.
If no horses around, try swordsmen with drill promotions. If you start next to a warmonger or England, you need to change your strategy, so consider taking one or two Authority policies (not really needed, but helps).
EDIT2.
BTW, I don't know if this works, but I think declaring war is enough to force spies to flee.