Is it hard for Brits to speak in American accents?

Depends how good your ear for  accents is.  Americans seem to have difficulty separating different British accents, but there are exceptions: Peter Dinklage the actor from New Jersey is capable of doing 'regions' of regional accents well enough to fool those who live there!

Think about it. Americans are the ones with accents. It's the English language.

No. British actors do it all the time. It is easier for Brits to speak with American accents than it is for Americans to speak with British accents.

It is, unless they can stop laughing.

Kenneth Branaugh, Emma Thompson are very good. David Tennant is okay. I like his Scottish accent better as Uncle Scrooge. Hugh Laurie's (House) American accent is very good. 

Most people find it hard to change their accent (whatever it is) for short periods of time. Like Mel Gibson murdering laland scots (otherwise he did a creditable job as Wallace)

(Yes, I know, he's australian not british, but he did a reasonable job as an american in lethal weapon.)

I can I think quite well. It's easier to do others though, I'm good at Pakistani.

Apparently.  Several years ago I went to a British play performed in the Soho District of London.  One of the roles was supposed to be an American detective.  The actor tried to do an American accent.  It was horrible.  The director would have been better off in switching that character to be British and forget about an American accent.

Probably not. Besides, mimicking an accent is easy but can be offensive

Actor Anthony Hopkins does an excellent job with all accents.