Note the very first sentence of UNGA Res 181, which makes plain that the Partition Plan was being brought *to* the UN *by* the Mandatory Power. So the UN wasn't "creating a state", it was merely consenting to the Mandatory's own plan to partition its Mandate into two successor states. The Mandatory definitely had that authority, subject *only* to the "consent" of its supervising body, which in 1947 was.... the UNGA.
Egypt postpones verdict in retrial of Al Jazeera journalists to Aug. 29 (Reuters)
from the article: Europe, stop appeasing Israel: Settlement goods are not 'Made in Israel'