WASHINGTON (Reuters) - The U.S. derivatives regulator warned Thursday that uncertainty over Britain’s exit from the European Union is having a "substantial" impact on some U.S. entities and markets. The Commodity Futures Trading Commission said the ongoing uncertainty about a Brexit outcome could create instability in the global derivatives market. In a statement, CFTC Chairman Christopher Giancarlo urged the EU and Britain to settle terms "in a manner that provides sufficient legal and..