Ha ha, well Vicky and I disagree on this one.
For IGS (not the forthcoming PSW scheme), for me it's clear that you can use various definitions of the date you successfully completed your course, the most generous of which is the end date of the visa you had for that course (assuming you had the normal allowance of +4 months). I know Vicky feels its risky to rely on this definition (scroll back in this thread for our exchange of views), but I still don't really know why you feel so strongly about this Vicky, and I have never seen a refusal on this basis. I work at a university, and for an independent student advice agency, and can only confirm that it is absolutely standard and normal across the whole UK HE sector for students to apply to IGS on this basis of "successfully completing their course" on that date. My previous posts had links to the UKBA website and the IDIs which both endorse the definition. Anyway I have never seen or heard of a refusal.
However, for the new PSW scheme, which is due to launch for worldwide applications on 30 June 2008, it does look like the date will be specifically defined as the date you were notified in writing of your result. This has not been formally clarified in any of the published information, but I had it last week on very good authority that unlike under IGS, the idea is that the date will now have a proper single definition.