As a graduate, you can state the university that you graduated from i.e. Imperial College London and upload a graduate transcript/degree certificate instead of the Acceptance of Studies Letter.
For "Course level", you should select the level of study that you attained. For "Year of study", please select "3rd year onwards" regardless of whether your degree lasted three years.
In addition, we request an e-mail with:
the Application ID that you can find once you make your account;
any evidence of internship or employment (in the form of an attachment);
if you plan further study any evidence of acceptance at a UK higher education institute;
either confirmation of the type of visa you intend to move to;
or evidence of the visa if you have already obtained it via https://www.gov.uk/evisa/view-evisa-get-share-code-prove-immigration-status. You can attach the downloadable output in your e-mail once you have followed the instructions.