Start with a regal invitation, like this one with gold gilded edges by Martha Stewart. Mimic the official Royal Wedding invitation by choosing a crown motif. Begin your invitation with, "The Lord Chamberlain is Commanded by...", just as the Royal Invitation states.
Decorate with bunting that displays the Union Jack -- Etsy shop Loopsies
Serve Eton Mess to guests. Prince William attended Eton College, so a dessert that makes reference is perfect -- BBC Good Food
Being a traditional English breakfast treat, scones are a must-have at the Royal Wedding Watching Party -- Pinterest
Serve an assortment of teas in pretty teapots and teacups. Be sure to sip, not slurp! -- Martha Stewart
Place vases of English garden roses with fancy feathered accents in the party space -- The Knot