LONDON — Tens of thousands of music lovers flocking to the outskirts of Paris for the Rock en Seine festival this week will be treated to British headliners including Loyle Carner, Fred Again and Olivia Dean. But the U.K. star power masks another change — the diminishing number of grassroots U.K. musicians performing at the continent’s festivals this summer. The reason? An increase in complex and costly paperwork facing British artists as a result of Brexit. According to research from the Best for Britain think tank, the representation of British artists at Rock En Seine has increased 24 percent compared to…