LONDON (Reuters) - British Prime Minister Theresa May said on Monday she was  ambitious and positive  about the country s future and its negotiations to leave the European Union, describing the talks as being conducted in a  constructive spirit .  I am ambitious and positive about Britain s future and these negotiations,  she told parliament.  If we are going to take a step forward together it must be on the basis of joint effort and endeavor between the UK and the EU.  