For what its worth, and maybe Stick or another one from UK can weigh in, I believe the Austins/Morris/Minis of 1959-mid 1960s had copper brake lines as original, but I believe they eventually changed to steel. As I recall the reasoning at the time was that the copper can "work harden" and become brittle and break. Since these cars had single braking systems this was a catastrophic failure. I had a 1966 Cooper S 1275 and recall this was a subject matter at the time.
Of course 1960's copper isn't exactly 2017's copper/nickel so maybe these new lines are an improvement...
Personally I don't think rust on a brake line is a major issue (compared to other parts of a car and its body) so the OEM steel ones should be fine.