“Mr. Thipps, touched by this sympathetic interest in the younger son of a duke, took the liberty, on their return to the sitting-room, of offering him a cup of tea.”
Dorothy L. Sayers, Whose Body?, pg. 22
Lord Peter is the younger son of a duke, so he comes from wealth and privilege. He may not give a second thought to coming down into these cases and investigating because he is so used to having his way.