When this started, most people had wisely run the other way.
John was apparently not one of the wise ones.
Sherlock had run towards the mess rather than away from it (John rather suspected that was due to Mycroft's earlier visit in which he explicitly forbade their interference).
So now John was chasing along after Sherlock as the genius idiot ducked behind rubble while trying to get a look at the aliens having a war in the middle of Greenwich University. One had a cartoonish hammer, a red Superman cape and was internationally recognizable. The other was coal black and flinging some sort of reddish black substance that ripped through the historic buildings as if they were clay. Oblivious to the mortal peril, Sherlock turned to grin at John with childlike glee as the doctor caught up and crouched beside him. The ground shook as Thor's hammer flew at his opponent, but the creature vanished and the hammer ricocheted instead to the corner of Sherlock's coat. The detective flinched away, but his coat didn't budge and now the God of Thunder was striding toward them. Finally, Sherlock had the sense to look worried and might even have blanched.
John sighed and stood, hefting the offending weapon while placing himself between the god and Sherlock. Thor blinked.
John handed the weapon back.