On CSPAN, when our esteemed congressmen were discussing Brexit, they constantly referred to the British as Brits. But I'll bet ya a million bucks, if one of them referred to the Japanese as a Japs... OMG....it would be the end of the world. So what's the difference? There is none. But there is. But there's not. Its all stupid.
The differences are [1] Brits is a common term the British call themselves Brits and do not consider it derogatory. [2] Brit is a word in common usage. [3] The word Jap was a derogatoryterm used during WW2 Here is a link for you 10 Things Brits Say…and What Americans Think We Mean http://www.bbcamerica.com/anglophenia/2012/08/10-things-brits-sayand-what-americans-think-we-mean
British people have a hard time with slang. Their minds are very uncreative and all they can think to do is start to say the word and leave off the last syllable and sometimes replace it with an "ie" like they call cigarettes ciggies. That's about it. That's as far as a British mind can go. And they think it sounds cool!