Start with the campus stuff that unlocks everything else
The Welcome Week brochure says the key first-week teams are based in Senate House, Paul Webley Wing. That covers the jobs most likely to slow the rest of your week down: collecting your SOAS ID card, sorting fees, enrolment or visa details, and finding the Student Hub or IT support before you need them in a hurry.
SOAS also says Student Ambassadors in purple bibs are there during Welcome Week, so do not waste half an hour pretending you can infer a campus map from vibes alone.