If nordost is your favourite, check out PJ hi-fi they have 17 meters of Nordost Flatline Gold bi-wire for £195 in the special offers section that's £11.47 per meter
Great cable I use it for my fronts
If you want to spend lots of money, then Nordost it is. That QED Qontour looks promising. But for 20m, the gauge of the wire is more important than the brand name. I suggest 14AWG as the maximum, the lower the better.