DITCHLING, England — In January, Michel Roux Jr., a London chef who is on the tasting committee that recommends wines to be served by the royal family, made a suggestion that would have been unthinkable not long ago. Wouldn’t it be nice, Mr. Roux asked a reporter for The Telegraph, if at the wedding reception for Prince William and Kate Middleton the glasses were filled not with Champagne but with English sparkling wine?