i want to install a basic ceiling extractor fan but do not want to wire it to the light rose box and would prefer to have a standard power lead to it plugged into a normal socket..is this ok?
That really depends how you define "ok".
A 'normal' socket is meant as a point where moveable equipment can be plugged in as and when it is needed.
To me, using a socket as a point for permanently wiring fixed equipment (regardless of how basic or standard it is) and tacking the flex around the walls is the hallmark of a bad job - you wouldn't expect to keep finding random plugs with wires disappearing off here there and everywhere in a new house, would you?
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