Copper brake pipe has been available in the Uk for many years. The correct pipe is a 3/16th inch OD with a thick wall giving about a 1/16" ID. This is plenty strong enough to handle brake system hydraulic pressures. yoeddynz is correct about ensuring suitable support for the tube as copper does work harden.
I have heard a few stories of brake failures when people have used 3/16th OD copper tube used for pumbing etc, this is thin wall and can fail.
Kunifer pipe is good, i've used both copper and kunifer types with no issues in 25 years of home made brake pipes......A good flare is of course essential ands it is well worth practicing a few flares if you are not familiar with the process before comitting to your final pipes.
Keep it safe......
Tony
HA21