haha good luck AccrossTheGlobe. You have been provided with many many choices here. I agree with most of them, except for Athens. If you do decide on Athens, spend no more than two nights.
My personal itinerary would be: London, Edinburgh, Amsterdam, Brugges, Paris, Berlin, Krakow, Prague, Budapest, Salzburg, Munich, Interlaken, Venice, Florence and Rome. If cost isn't an issue, I would also chuck Iceland in there for four or five days. The flights aren't that expensive from London. Only go to Iceland if you are into nature, not many castles there.
If you are travelling Sept - Dec, I wouldn't bother pre-booking hostels. You can just rock up when you like. Try to come up with a logical itinerary which avoids backtracking. Also, remember, sometimes flights are cheaper and quicker than trains. Keep that in mind.