The Pope was iudex totius ecclesiae. The principle was to be found in a constitutum of Pope Silvester I: "No one will judge the first see" - Nemo iudicabit primam sedem. It was assimilated into the False Decretals of Isidore, and later confirmed by Pope Nicholas I as canon law, and included by Gratian in his Decretum Gratiani as Pars 2 Causa 9 Quaestio 3 Capitum 13.
C. XIII. Prima sedes nullius iudicio subiaceat. (Item Innocentius Papa.)
Nemo iudicabit primam sedem, iustitiam temperare desiderantem. Neque enim ab augusto, neque ab omni clero, neque a regibus, neque a populo iudex iudicabitur.
It was also included in Pope Gregory VII's Dictatus Papae as clause 19: That he himself may be judged by no one.
The right of appeal to the Pope in Rome conflicted directly with the King of England's own right to be the fount of all justice in his own kingdom, and the highest authority in England? Where did the buck stop, With the King or with the Pope? The king's coronation oath defined the king's role in this matter and his promise to the people: that the church of God and the whole Christian people shall have true peace at all time by our judgment; Second, that I will forbid extortion and all kinds of wrong-doing to all orders of men; Third, that I will enjoin equity and mercy in all judgments.
The Constitutions of Clarendon attempted to answer that question directly in Clauses 4, 7 and 8, and others.
Becket challenged these principles by the very many excommunications he delivered to various bishops, barons and others. at Vézelay and other places. They would all have to make an appeal to Rome, to petition the Pope and obtain absolution from him. Not even the King would be able to deal with their cases. Becket was clearly trying to force the King's hand, by a clear demonstration and exploitation of a weakness in the Constitutions' own legal position, by exposing a contradiction and flaw in its formulation. And neither could the Pope ignore his situation as a result.
Statute in Restraint of Appeals - Wikipedia
The Act, drafted by Thomas Cromwell on behalf of King Henry VIII of England, passed in 1533, which forbade all appeals to the Pope in Rome on religious or other matters, making the King the final legal authority in all such matters in England, Wales, and other English possessions.
Council of Sardica [A.D. 343 OR 344]
Medieval Sourcebook: Council of Sardica: Canon V
Canon V [Greek Version]
The famous discussion between the African bishops and the Bishop
of Rome, on the subject of appeals to Rome.
Antoine Charlas (1684). Tractatus de libertatibvs Ecclesiae Gallicanae apud Matthiam Hovivm. pp. 587–.
Gregorio XIII (Papa); Justus Henning Boehmer (1839). Corpus juris canonici: Decretum Gratiani. Secunda Pars Causa IX Quaest. III C. 13 : sumtibus Bernh. Tauchnitz jun. Col 522
J. H. Burns (17 October 1991). The Cambridge History of Medieval Political Thought C.350-c.1450. Cambridge University Press. pp. 286–. ISBN 978-0-521-42388-5.
Wilfried Hartmann; Kenneth Pennington (2012). The History of Byzantine and Eastern Canon Law to 1500. Apellatio: CUA Press. pp. 72–3. ISBN 978-0-8132-1679-9.
Healy, P. (1912). Council of Sardica. In The Catholic Encyclopedia.
Papi, H. (1907). Appeals. In The Catholic Encyclopedia.
For the rights of the Church and its supreme head, the pope, to receive appeals in ecclesiastical matters. For these and other similar questions see POPE, PRIMACY, COUNCILS,GALLICANISM, ECCLESIASTICAL FORUM.
Wilfried Hartmann; Kenneth Pennington (2008). The History of Medieval Canon Law in the Classical Period, 1140-1234: From Gratian to the Decretals of Pope Gregory IX. CUA Press. pp. 292–. ISBN 978-0-8132-1491-7.