Mike wrote:

One question though - how come a lightbulb isn't a fixed appliance ?

It is - or rather a luminaire is fixed equipment. Isolation
requirements for the circuit are met at the consumer unit by way of an
MCB and/or main switch with provision for locking-off. The additional
specific requirements for a fan arise because of the presence of a
motor. [See pp. 34-35 in the OSG.]

BTW isolators don't, in general, have to be double-pole, except in
TT-earthed installations, and except for the main switch(es) of a
single-phase domestic installation. (In 3-phase installations it's very
common practice for isolators to be 3-pole with a solid removable
neutral link.)

--
Andy