Contrary to common belief, the regs themselves do not require such an isolator. The manufacturer's instructions may well 'require' an isolator (and quite probably also a 3A fuse) and, until a few months ago, the regs did require compliance with manufacturer's instructions. However, even that requirement has now been relaxed.I think sparky fitted isolator as regs- so that person must be able to isolate (on 3 poles) the fan when needs be.
Kind Regards, John