It differs at each University - I believe emails are assigned in early to mid September at my university.
If you just need the letter for Council Tax reasons, I'd suggest getting in touch with Student Services from your personal email account, as they normally produce standard letters for this simply saying that you are a student, the course you are registered on, and the start and end dates of your course. You probably won't need a student email for this, but it may be the case that you need to wait until you've officially enrolled in your course. Enrollment typically happens during Freshers Week or the first week of your course.