Sadly, many suppliers are supplying almost pure copper, which as stated, is unsafe, for brake and fuel systems. Now, failure is unlikely in the fuel system, as the working pressure is very low, provided that the pipe is secured very well to prevent vibration, and I don't think that we know that it would be illegal there. The reason for NOT using copper or Kunifer is because it reacts with E5 or E10 fuel, and results in metallic compounds being deposited downstream, in the carburettor or fuel injection system, potentially causing major damage. It is very unlikely that even E10 will dissolve its way through the wall of the pipe, and I certainly did not intend to give that impression.
By the way, the use of copper in brake systems is not specifically outlawed in the UK, probably because VOSA are too incompetent to see the problem. However, it is illegal under generic provisions of the C&U Regs, by virtue of the fact that it us unsafe, due to tolerating an inadequate number of pressure cycles. It does not need to be specifically outlawed, although it is outlawed in the US (who are usually quite lax about vehicle safety, look at some of their truly appalling hot rods and you will see what I mean), Australia, most of Europe and various other places. The suppliers are committing fraud by passing it off as "brake pipe", because it is not, and the BS specification that some of them quote is intented to deceive, because the spec is not about brake pipe. Beware, many well-known Mini suppliers are in fact still supplying copper, despite claiming that they do not.
As for the fuel system, I have not been able to find any reference to rigid metal pipe having to be specifically approved or marked, although it is a legal requirement for flexibles, so I suspect that it is probably ok legally. But, as a general rule, you should never, ever give or receive legal advice on an internet forum, you really must consult your own lawyer for that.
All of the potential problems go away if you just use rigid pipe of the type used on modern cars, as I said earlier. And, it will normally by 6mm or maybe 1/4", so check that your legally marked flexibles, properly rated for E10 fuel, will fit correctly. (If the proposed supplier can't confirm compatibility, go elsewhere.) Usually injection systems also have an 8mm return line.
Oh, and don't ever use the plastic-coated steel pipe which is commonly available, and highly praised by certain people. It is potentially lethal, because the coating becomes damaged slightly, and salty water gets in, causing very localised but deep pitting, which suddenly turns into a pinhole, causing sudden failure. Untill the point of failure the pipes look to be in good condition. That stuff ought to be illegal, as it has very dangerous and undetectable failure modes. It is as useless as old-fashioned underseal, for exactly the same reason, it only works when perfect, and accelerates hidden corrosion once it is cracked ever so slightly.
