LONDON (Reuters) -British American Tobacco's admission that its U.S. cigarette brands will be worthless within decades has ramped up pressure on the company to prove it can better compete in alternatives like vapes. Earlier on Wednesday, BAT put a 30-year lifetime on some U.S. tobacco brands' value, taking a $31.5 billion noncash impairment. The move marked the first time a tobacco company has acknowledged that hugely profitable brands have no economic future.