Catalyzes the formation of UDP-glucose, from UTP and glucose 1-phosphate. Is highly specific since it cannot use other NTPs such as dTTP, CTP, ATP, and GTP, and other sugar-1P such as GlcNAc-1P, Gal-1P, and Man-1P, as substrates. Has probably a central and essential role as the substrate supplier for galactolipid synthesis; galactolipids are major constituents of the photosynthetic thylakoid membrane and important for photosynthetic activity