Katy Perry (41) returns with a bang on the British stage: at the Blenheim Palace Festival in Woodstock, Oxfordshire, the pop singer sets a spectacular headliner set with her "Out of Office" tour – and, according to Mirror, distributes plenty of side-lifts to her ex-partners by the way. In front of the impressive backdrop of the castle grounds, a hit chases the next one, from "California Gurls" to "Wide Awake" to "Dark Horse". But for the biggest…