Whatver other changes you can make over there, the two most important and vital are:

Update the Constitution. An 18th-Century document designed to facilitate oligarchical rule by the wealthy and educated is no longer fit for purpose in the 21st Century.

Place the President, or at least the Executive power, in the House, remove the Presidential Veto and limit the power of the Senate. Our system may not work as well as it should, but the fact that our Executive -the PM and Cabinet - are part of and thus accessible and accountable to the directly-elected representatives of the electorate does put a severe brake on autocratic tendencies at the top. Even Thatcher, of accursed memory, with her legendary majorities, couldn't have got away with a quarter of what Trump did.