Who was the last Prime Minister to spend most of his premiership on the benches in the House of Lords?

It is a bit of a trick question.  The answer is Labour Prime Minister Clement Attlee (1945-51).  The chamber of the House of Commons was destroyed by enemy bombing on 10 May 1941.  The decision was taken to re-build it on the same basis as the old chamber.  However,  the new chamber was not completed until the middle of 1951.  Until then, the House of Commons sat in the chamber of the House of Lords.   The Speaker’s Chair was placed at the opposite end of the chamber to the Throne, so the Government Front Bench was what is now the Opposition Front Bench.

The House of Lords sat instead in the Robing Room (a large room overlooking Victoria Tower Gardens), with temporary division lobbies created each side of the room.  The Norman Porch (the porch at the top of the steps leading from the Sovereign’s Entrance – the steps where visitors begin their guided tours) served as the ante-chamber, where peers could meet inforrmally outside the chamber.

The members of the reforming Labour Government under Attlee thus spent most or all of their ministerial careers sat on the red benches of the Lords.  Within a few months of MPs moving into the newly-rebuilt chamber, there was a General Election and Winston Churchill was returned to office.