@coldjoint,
I really can't remember that Obama told a political party of a foreign country whom to choose as next Prime Minister.
The UK can't neither - that's why it's called today even by Conservatives (and in the linked report) a "dramatic break with diplomatic convention".
But even if it's Boris as Trump wishes: the new prime minister will inherit the same parliamentary arithmetic as the current prime minister. And there are a few more There are some inescapable facts that cannot be wished away, even not by Trump e.g. the terms of our current extension, the current Speaker ...)