LONDON—Barclays reported on Thursday a doubling of third-quarter profits, beating market expectations, as it followed Wall Street rivals in reaping bumper investment banking fees from a surge in advisory mandates and equities trading. The British bank posted profit before tax of 2 billion pounds ($2.8 billion) for the July-September period, better than the 1.6 billion pounds average of analysts’ forecasts and twice the 1.1 billion pounds it made in the same period a year ago. The strong performance showed the value of Barclays’ U.S. business, where it makes nearly half its income, and British banks’ balance sheets holding up amid the pandemic with the rest of the sector due to report over the next two weeks. Barclays’ advisory and equities business had a record performance in the first nine months of the year, the bank said, driving a return on equity for the overall investment bank of 16.4 percent compared …