Location of BGT cull

[Peter Pedant]

Simon and his lot were busy cutting the throats of various hopefuls,
from 140 to 40

and the setting was a main room/throne room of some grand palace/place

in the back there were various gold objets or gold plated possibly

and high up on the left hand wall possibly a portrait of a young victoria

and when they were led out to the as grand staircase
there seemed to a gold patriarch witnessing their rejection or perhaps blessing their failure

any idea of the palace/ location ?