The Garden Party is an excuse for you to spend lots more money on not very much, but it's part of the whole graduation experience. You pay your £10.50 (or whatever it is this year), go into a marquee (or somewhere else if it's raining/full) on Lower College Lawn, get a free glass of bubbly, a couple of small sandwiches with the crusts cut off, and if you're lucky one or two *miniature* Fisher and Donaldson delights. Not really worth £10.50, but never mind.
The General Council pish is (I'm a little hazy on this) so that you get a free Alumnus magazine every year and get to have a say in the running of the University in a very loose sense.
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...but then again, that is only my opinion.