After doing some research it appears the UAOC's claim to autocephaly relies on the Tomos given to the Polish Orthodox Church. Now before anyone starts, we all know the "Polish" Orthodox Church is comprised almost entirely of Ukrainians and Belarusans. So it seems the UAOC which emerged in Western Ukraine (which was not part of the Soviet Union at the time of the Tomos of 1924) during WWII, saw their right to autocephaly as a logical extension of the grant to the Polish Orthodox Church. If the Ukrainian Orthodox of Poland, no longer part of Russia, were autocephalous, the Ukrainian Orthodox of Ukraine, also no longer part of Russia, should be autocephalous too. They also see the autonomy of the Metropolitans of Kyiv and the Metropolitans of Halych as giving them precedent. Many of the UAOC members were former Greek Catholics which also probably didn't endear the MP to them. Finally the support given them by the Polish Orthodox Church (Metropolitan Dionisij ordained them bishops) encouraged them.
+GREGORIOS VII, by the grace of God Archbishop of Constantinople -the New Rome and Ecumenical Patriarch
The Holy Orthodox Church in the God-Protected Polish State, endowed with an autonomous system and administration and proving its firmness in faith, zealousness by charitable works has requested our Holy Apostolic and Ecumenical Patriarchal See to bless and confirm its autocephalous administrative system, considering that in the new circumstances of political life, only such a system can satisfy and guarantee its needs.
Examining this request with love, taking into consideration the structions of the holy canons, which have established that the system of church affairs should correspond with the political and community forms (IV Ecumenical Council, canon 17, VI Ecumenical Council, canon 38), as well as the reasoning of Photius: "It is acceptable that laws which relate to church affairs, and especially parish matters, should correspond with political and administrative changes", from another point of view, bowing before the demands of canonical obligations, which impose upon our Holy Ecumenical See concern for Orthodox Churches, who are in need; considering also the fact, which is not contradicted by history (for it is recorded that the first separation from our See of the Kyivan Metropolia and the Orthodox Metropolia of Lithuania and Poland, dependent upon it, as well as their incorporation within the Holy Moscovite Church was accomplished contrary to canon law, as also all that which was agreed upon regarding the full church autonomy of the Kyivan Metropolitan, who at the time had the title Exarch of the Ecumenical See), We and our Holy Metropolitans, Our beloved brothers and co-workers in the Holy Spirit, considered it our obligation to give ear to the request presented to Us by the Holy Orthodox Church in Poland and to give Our blessing and approval to its autocephalous and independent administration.
As a consequence of this conciliar decision, following the guidance of the Holy Spirit, We have decided: to recognize an autocephalic administration for the Orthodox Church in Poland and give our blessings to this, so that from this day on It may be governed as a spiritual Sister, and decided Its affairs independently and in an autocephalic manner, according to the regulations and unlimited rights of other Holy Autocephalic Orthodox Churches, recognizing as its Supreme Church Authority, the Holy Synod, composed of orthodox canonical bishops in Poland, whose president shall at all times be the Blessed Metropolitan of Warsaw and of all Poland. To preserve and canonically maintain united with Our Holy Apostolic Ecumenical Patriarchal See, as well as with all other Autocephalic Orthodox Churches, We mention here the obligations which every Metropolitan of Warsaw and of all Poland will have: ie. to inform, according to the regulations of the Holy Orthodox, of his election and enthronization by an enthronization letter Our Great Christian Church as well as all other Autocephalous Orthodox Sister-Churches: To retain everything related to a firm maintenance of the Faith and Orthodox piety, as well as all that is commanded by the holy canons and regulations of the Orthodox Church; to also commemorate in accordance with regulations in the Diptychs the name of the Ecumenical Patriarch and other Patriarchs as well as the Hierarchs of other Autocephalic Churches. In addition to this we decree, that the Autocephalous Orthodox Sister-Church in Poland must obtain its Holy Myrh (oil) from Our Great Christian Church. We advise at this time, that in matters concerning church order and in matters of a more general nature, which are beyond the jurisdictional limits of every Autocephalous Church acting individually, that the Blessed Metropolitan of Warsaw and of all Poland to apply to Our Holy Ecumenical Patriarchal See, through whose mediation union with every Orthodox Church, "... rightly teaching the word of truth" and request authoritive opinions and help from Sister-Churches.
Having carefully reviewed and considered all this at canonical meetings of the Holy Synod on the sixth and eleventh of November, 1924, We have entrusted, after the approval of the Synod, this Synodical and Patriarchal Tomos, accurately and unalteredly copied, as it is recorded in the Codex of Our Great Christian Church, to the Blessed +DIONISIJ, Our beloved brother and co-worker in Christ, Metropolitan of Warsaw and of all Poland and President of the Holy Synod of the Autocephalic Orthodox Church in Poland.
May the Lord God strengthen unto the ages, by the grace and merits of the First Great and Supreme Pastor, Christ our God, the Autocephalous Sister-Church in Poland, so fortunately organized, may He raise and increase everything in it to the glory of His Holy Name, for the benefit of Its pious flock and for the joy of all Autocephalous Orthodox Sister Churches.
In the year of Our Lord 1924, November 13th.
The Patriarch of Constantinople, +GREGORIOS VII (Approved).
The Metropolitan of Kiza, +KALYNYK
The Metropolitan of Sardia and Pisidia, +HERMAN
The Metropolitan of Nicea, BASIL
The Metropolitan of Chaldea, +JOAKIM
The Metropolitan of Philadelphia, +PHOTIUS
The Metropolitan of Derkos, +CONSTANTINE
The Metropolitan of Syliria, +EUGENE
The Metropolitan of Brus, +NICODEMOUS
The Metropolitan of Rodopolia, +CYRIL
The Metropolitan of the Princes Islands, +AHAFANAHEL
The Metropolitan of Neocesarea, +AMBROSIOS
The Metropolitan of Anea, +THOMAS
The Ecumenical Patriarchate, November 13, 1924
Chancellor HERMAN, Metropolitan of Sardia