Just keep them short (so impedance matching barely matters), and use digital interconnect whenever you can. CPC is a good source. Speaker cable needs to be a bit thick (large currents). Make sure you aren't using composite video anywhere (use Component, RGB or S-video wherever you can). I like the flat Scart cables (don't worry, each wire is a screened coax); they are much easier to route than the stiff snake style. it's worth getting a Sky box with digital sound out (far better than DAB); i.e. Sony from ebay or Sky+.
Forget cable brands. It's just long thin bits of copper. Brands are a mechanism for the retailer to make as much margin on the cables as he does on the brown boxes by selling 50p cables for £20. It works like this: you sell the "£20" branded cable to small retailers for £12.50 and to Dixons for 75p. They both sell it on at around £20. If Dixons finds a small retailer undercutting them, they will switch brands.
If you are, unlike me, a believer, "spend about 10% of your total system cost on interconnects of a reputable brand".