Sales of Rolls-Royce and Bentley motor cars in the United States last year rose to 1,208 from 1,155 in 1986--an increase of almost 5%, giving the company its best year since 1976. The British car maker said worldwide sales rose by 7%. In the United States, the largest market, 213 of the automobiles sold were Bentley models--double the previous year's number. The surge in Bentley sales was attributed to the introduction of two sporting performance sedans--the Bentley Eight and Bentley Mulsanne S. Demand for the company's flagship models, particularly the Rolls-Royce Silver Spur long wheelbase sedan and the coach-built Corniche II convertible, was also strong. Sales of the hand-built convertibles, at 301, were a record. In Britain, 842 cars were purchased, a 4% improvement on the previous year, while Europe and other world markets registered an increase of 11%.