There's a few sellers on ebay but they all appear to be in China. It'll be exactly the same product you will get from a UK seller and a bit cheaper. Just search for black RGB SMD5050 and it should find them.
you want the 300 led per 5 meter, rather than the cheaper 150led per 5 meter.
The waterproof version has a gel coating that is useful for using the leds in areas it might get wet, like a kitchen. No real advantage for use in a PC case really. If there was a way to 'frost' the coating, it could help to diffuse the light, but I don't know how you could go about it. It's a little more expensive than the non waterproof.
I'm going to go for the black strip as I think it will match in with my black case interior. The white tends to stand out a bit. It's probably not going to be seen behind the window panel, but i'm a bit of a perfectionist
. It'll also blend in better on the back of monitors, so will probably use some of the leftover strip for that.