The Verse in the Question.:
“yo brahmaaNa.n vidadhaati puurva.n | yo vai vedaa.nshcha prahiNoti tasmai .ta.n ha devaM aatmabuddhiprakaashaM | mumuxurvai sharaNamahaM prapadye |” (Svetaswatara Upanishad 6:18).
“Seeking Liberation, I take refuge in the Lord, the revealer of Self-Knowledge, who in the beginning created Brahma and delivered the Vedas to Him”.
Svetaswatara Upanishad clearly says Rudra is the only Brahman without any second entity.:
“eko hi rudro na dvitiiyaaya tasthurya imaa.nllokaaniishata iishaniibhiH |
pratyaN^ janaastishhThati saJNchukochaantakaale sa.nsR^ijya vishvaa bhuvanaani gopaaH |” (Svetaswatara Upanishad 3:02).
“Rudra is truly one; for the knowers of Brahman do not admit the existence of a second, He alone rules all the worlds by His powers. He dwells as the inner Self of every living being. After having created all the worlds, He, their Protector, takes them back into Himself at the end of time”.
It also says clearly that Rudra gave birth to Brahma..:
“yo devaanaaM prabhavashchodbhavashcha vishvaadhipo rudro maharshhiH | hiraNyagarbha.n janayaamaasa puurva.n sa no buddhyaa shubhayaa sa.nyunaktu |” (Svetaswatara Upanishad 3:04).
“He, the omniscient seer Rudra, the creator of the gods and the bestower of their powers, the support of the universe, He who, in the beginning, gave birth to Hiranyagarbha—may He endow us with clear intellect!”
Both Dakshina Murti and Sharabha Upanishad clearly states it's Rudra who delivered the Vedas to Brahma.:
“sargaadikaale bhagavaanviri~nchirupaasyaina.n sargasaamarthyamaapya | ” (Dakshinamurty Upanishad 1:20).
“At the beginning of creation, Brahmâ the Lord, having worshipped S’iva, attained power to create and was delighted at heart”.
Note.: Brahma created the Universe from the knowledge of the Vedas.
“prabhu.n vareNyaM pitaraM mahesha.n yo brahmaaNa.n vidadhaati tasmai |
vedaa.nshcha sarvaanprahiNoti chaagrya.n ta.n vai prabhuM pitara.n devataanaam.h |
mamaapi vishhNorjanaka.n devamiiDyaM yo.antakaale sarvalokaansa.njahaara |
sa ekaH shreshhThashcha sarvashaastaa sa eva varishhThashcha |” (Sarabha Upanishad 1:2-3).
“I am saluting that primeval God who is the Lord, who is the best, who is the father of the world, who is the greatest among gods, who has created Brahma, who gave all Vedas to Brahma in the beginning, who is the father of Vishnu and other devas, who merits praise, and who at the time of deluge destroys the world. He is the only one who is greater than every body, who is the best and who rules over others”.
Again, it's Svetaswatara Upanishad only which clearly says us take refuge in Rudra especially his Dakshina (Southern) face (Dakshina Murthy) for liberation.:
“ajaata ityeva.n kashchidbhiiruH prapadyate | rudra yatte daxiNaM mukha.n tena maaM paahi nityam.h |”(Svetaswatara Upanishad 4:21).
“It is because Thou, O Lord, art birthless, that some rare souls, frightened by birth and death, take refuge in Thee. O Rudra, may Thy southward face (dakshinam mukha) protect me forever!”
Conclusion.: So, the Lord mentioned in Svetaswatara Upanishad 6.18 is none other than Bhagawan Rudra who is the only Brahman present in different different forms doing different different duties.
I hope this clarifies all your queries. Prd..