Why are all the Characters of JO has a British accent??!
Not all. Only Imperial officers and politicians(even Jedis) who have survived from the last days of the Old Republic like Mon Mothma(I guess it strengthens the image of British Empire and the fact that whenever someone makes a movie about Roman Empire which Galactic Empire follows symbolically, they hire British actors to play senate members).
However, heroes and rebels in general speak with an American accent or in case of Wedge, for example, like Scots.