// yes, it's been done, Marquess of Salisbury was PM// (*)
and he was the last.
George V could not decide whether to ask Lord Curzon ( "I am george nathanial curson and I am a very superior person" that one) to form a ministry in 1935 and was advised by Lord Stamfordham (**) Di's grandfather, not to. The PM it was felt should be questioned on policy by the commons and that was not possible
Mossis Thotcher tried to have a Foreign Secty in the Lords - Lord Carrington - but as he lost the Falklands, the experiment was not deemed a success.
(*) Robt Gascoyne Cecil had a son Ld Edward Cecil who wrote memoirs of an egyptian official. When the Boer War broke out 1899, there was no money to pay the army in Egypt. Nichty Nochty - the cupboard was bare, down to their last piastre. Lord Eddie went down to Baring Bros in downtown Cairo and wrote out a personal cheque for £1m. (between a hundred million and a billion nowadays) and one of the Baring bros murmured - "and how do we know this cheque will be honoured my lord?"
and lord eddie answered - because my father is prime minister. The money was made available.
(**) that is stam-fuddum - played well by Bill Nighy in The Little Prince.