Search is you friend. Benjamin was never "merely a London barrister". He was a famous barrister, a QC, and the author of a legal textbook which is still in use today. He represented Canada in several important cases, then became a key adviser both to the privy council and to the judicial committee on the House of Lords. Benjamin argued that the Canadian central government was formed only for the purpose of protecting Canada from the United States, and that all other powers rightfully resided in the provinces. To this day the Canadian government is more of a true federation than our government and this is largely the legacy Benjamin's legacy.
In 1867, when the North America Act was written, he was merely a London barrister. His influence over the Act has been widely stated by the Southron contingent, and disproven by scholars. Oh there is no disputing that Benjamin did go on to a distinguished legal career, or that he took silk in 1872 or that he published his book on property law in 1868. But he didn't play a part in the creation of the Canadian constitution.