Roskill's oldest brother was Sir Ashton Roskill QC; another older brother Stephen was a Royal Navy officer and historian. He was educated as an exhibitioner at Winchester College and went then to Exeter College, Oxford, where he graduated first class with a Bachelor of Arts in 1932, winning an honorary scholarship in modern history.
Roskill's oldest brother was Sir Ashton Roskill QC; another older brother, Stephen, was a Royal Navy officer and historian. He was educated as an exhibitioner at Winchester College and then went to Exeter College, Oxford, where he earned his first-class degree in 1932 with a Bachelor of Arts in modern history.