It isn't even a question of Trump's abilities as a negotiator, though -- he, or anyone else in the position of PM, would still have been bound by the Lisbon Treaty and the UK Constitution, as well as political realities. The first two essentially force the UK to wait two years from notification to leave anyway; the second means that no matter what the UK PM says or does, the EU would have stuck to its general position.
Of course, since Trump's suggestion recently was to "sue the EU", one might also question whether he'd really be an improvement, but the fact of the matter is that no one person could make much of a difference to the present situation.