After the Ashre and Mizmor 20, “Uva Letzion” and the special Kedusha that accompanies it (known as “Kedusha D’Sidra”) are recited. Several reasons are given as to why our Sages included it in the weekday morning prayers. The Avudraham explains that it was intended for those unlearned people who would arrive late to the synagogue to be able to recite Kedusha, which they missed during the repetition of the Amida. The Levush (O.H. 132) writes that Uva Letzion was enacted because at the time, it was decreed by the non-Jewish authorities that sanctifying Hashem through Kedusha was forbidden. As such, it was placed after the Amida, at which point the authorities believed the prayer was already completed. Furthermore, the Kedusha is accompanied by an Aramaic translation and one reason was so that the aforementioned unlearned congregants would be able to understand what was being said, as Aramaic was the vernacular of the masses at the time. The Arizal explains that it is also recited in Aramaic so as to conceal the full extent of its holiness, which may be too much to bear. As well, Rabbi Moshe Zachuta says that since the angels’ function is to sanctify Hashem, they may become jealous, as it were, when we say Kedusha and thus it is recited in a language they do not comprehend.
The Rama (Orah Haim 132:1) explains that the Kedusha D’Sidra has the same status as the Kedusha recited in the blessings of Keriat Shema, and as such it may be recited by an individual without a Minyan. The HIDA (Kesher Gudal) concurs, and Rabbi Shalom Messas explains that this was also the custom in Morocco.
Summary: Uva Letzion and Kedusha D’Sidra hold deep significance and they may be recited without a Minyan.