Is this something you need in official writing? Because if not, you can look it up yourself on the university website (term/semester dates calendar). Just off the top of my head, if it's a three-year course, which starts in September 2008, then your degree end date will be June or July 2011 (whenever your university has it's official graduation for that year). I haven't actually seen anyone mention needing an official end date for their visa application in the past, but I could be wrong.
I'm not sure if it works in the same way for UK student visas, but when I applied for my US student visa last year, I just guessed the end date (December 2012... although having spent a semester there this year, I realised it would probably be closer to 2014 by the time I finished!). However, in actual fact, my visa was valid for the 'Duration of Status', so even if I was still enrolled as a full-time student after the date printed on my visa had passed, I would still legally be in the country on a student visa. It's only if I drop below the required class registration and am 'out of student status' (which I have now done because I've moved back to the UK) that the visa ceases to be valid.