The President's equivalent in the UK is the Queen as Head of State, so Buckingham Palace is equivalent to the White House
The Prime Minister is the head of the Government, whose official office and private residence are both at 10 Downing Street, which is where the Cabinet meet.
The seat of government as an institution is in Whitehall.
"Downing Street" indicates that it is the Prime Minister's office that is making this plea.
Like so many things, there is no direct equivalent. "Downing Street" and "The White House" are both (in a sense) abstracts representing the point from which political directives are issued, therefore #2 is the best.