Sia, who is known for hiding her face in public with her signature blonde wig, was more than happy to reveal her derriere in the brazen stunt.
Buchan joins as the upwardly mobile Sir Thomas More and Egan will play Tudor stalwart General Howard.
Scroll down for video 'Save your money, here it is for free! While Alfie seemed to enjoy himself on the trip, he couldn't resist making a joke about his ideal date, posting a picture of footballer Danny Welbeck with the caption 'Happy Valentines Day'.