First, there are many different british accents, some of which will be absolutely no use to you. You mean, I think a cultured public school type of accent. This is difficult to learn quickly. You would need years to even come near it, and probably never succeed, as language and accent are formed normally before the person is three years old. After that you are stuck... My advice is to just try and speak plain english in your own way. Don't lose your heritage.