Speaker cable suggestions for B&W 800 Diamond

Yes the Kimber 12TC is a good cable.
twenty meters is LONG length though. You would be bettter off placing your amps closer to the speakers for best results.
If you have a special installation and cannot move the amps i can understand the problem.

For that length I would certainly do the 12TC over the 8TC

Buying unterminated Kimber bulk will even be cheaper

Depends on what you want to spend for money, The kimber mentioned above is ok, however, Their is alot out there thats so much better for such fine speakers you are about to have!cheers!