
Yes. They were all Anglo-Normans. All of the guys involved, with the exception of the mayor of London, who was not Anglo-Norman but descended from the English, were close or distant cousins. When the Normans conquered the country, they spent a lot of time intermarrying, keeping power in Anglo-Norman hands.
The "Committee of 25", were a group of barons, who stood in opposition to King John. They were entrusted, by the terms of Clause 61, of the Magna Carta. They were,
Eustace de Vesci
Robert de Ros
Richard de Percy
William de Mowbray
Roger de Montbegon
John FitzRobert
William de Forz
John de Lacy
Saer de Quincy, Earl of Winchester
Richard de Montfichet
William de Huntingfield
Roger Bigod
Hugh Bigod
Robert de Vere
Geoffrey de Mandeville
Henry de Bohun
Richard de Clare
Gilbert de Clare
William D'Albini
Robert FitzWalter
William Hardel
William de Lanvillei
William Mallet
William Marshall II
Geoffrey de Say