If you want to to use British English then the standard is known as 'received pronunciation'. It is widely used in business, media, and academia and is understood across Britain and beyond.
I use a book entitled 'Get Rid of Your Accent', by Linda James and Olga Smith. It is accompanied by a CD and follows a very simple, yet effective, method.
Of course, there are other books available, but I have found this one to be quite suitable.