Written material won't help you learn an accent, but it will help you learn different spelling and common words that are different between American and British English. If you want to learn an American accent, any materials geared toward the TOEFL test will help. The British accent is a little more difficult to learn without a proper native British speaker. The IELTS is produced by a British company, but their materials use different accents to promote multicultural understanding. I don't personally know of any program that is completely taught in a British accent.