Look, we all know that Obama wants Britain to stay in the EU because it is in America's interest that Britain is in. It is inevitable that a certain amount of US pressure is going to seep into the debate, but it's very unusual for a foreign political leader to directly advise the citizens of another nation to "vote for such and such a person" or "not to vote for such and such a treaty", at least in the terms that Obama used.
I have no problem with Brits being furious at Obama. In fact, I encourage it. As long as it doesn’t lead to the usual America-bashing, of course.