in fairness, if you're trying to get all that done, and it sounds like you don't really know what you're at, maybe it's worth talking to an installer? you've got the right idea on some of it, you can have the central storage, xboxes will strem them for you if you mod them and get xbmc running on them, know then you'll need a dsiplay in every room to see what to select. also xbox360 can run as a media extender with no modding, but then you need a host pc runnin mce to stream from. with the modded xbox you can run off a nas and need no pc running Also an xbox can't drive a set of in wall speakers, you need an amplifer to drive in wlls, unless you get a pair with in built amplifiers
sky box will need a distribution amp in the attic or somewhere, run ct100 or equivalent, run at least 3 cables from the attic to whereever there will be a physical sky box, 4 is better, and then a run from attic to every room.
don't try and feed the output from a bluray player around the house, more effort than it is worth. Put one in the room you want to use, get a hdmi cable and hook it to tv. Decent quality long hdmi cables are expensive, and once you go over 3/4m you want to avoid cheap ass cables. if you want bluray in another room that bad buy a player. distribution amps for hdmi aint cheap...