LONDON—Trading volumes at major cryptocurrency exchanges fell by more than 40 percent in June, research showed on Monday, with a regulatory crackdown in China and lower volatility among the factors depressing activity. Spot trading volumes fell 42.7 percent to $2.7 trillion, with derivative volumes down 40.7 percent to $3.2 trillion, London-based researcher CryptoCompare’s data showed. “Headwinds…