Dan Starkey (in-universe)

An unnamed man demonstrated Strax's advice about watching The Day of the Doctor in 3D. The man developed a mild headache after wearing a pair of glasses, then rubbed his eyes whilst muttering "what was the point in this exactly?" The man also wore a t-shirt with "reverse the polarity" written on the front. (NOTVALID: Strax Saves the Day)

Behind the scenes
While he was left unnamed in Strax Saves the Day, it was pretty self-evident that Dan Starkey played himself in the respective scene.