Which colour wire is live in UK wiring?

In a flexible cable, the brown is the "line" voltage and blue is "neutral", often tied to ground at the mains panel.

In fixed cables, i.e., "behind the walls", the UK wiring standard changed in 2004, where it now MATCHES the flexible cable: brown is line, blue is neutral. Prior to that, blue, red or yellow were acceptable LINE conductor colors and black was neutral.