ceiling fan in council house - if want to keep it, have to show certificate

This work was pre part P, even to today's standards a ceiling fan from a lighting circuit would only require a minor works certificate and wouldn't be notifiable to building control. This wouldnt have been certificated 20 years ago. All I can imagine is that they don't wish to have an aging fixed appliance in situ or you have a jobsworth hitler to deal with. Either fight it, or maybe treat yourself your a newer one and get a minor works at the same time to keep them happy. If you are council there is little they can really do as you have a secure tenancy and they should prove that it is unsafe.

Usually with council properties you can sign a tenants own fixtures/fittings form, I'm pretty sure you're not only tenant to have their own light fittings. This form omits the council from any liability etc. It could be that there is no earthing conductor on the lighting circuit and the ceiling fan is a class 1 accessory (I know you've said its been rewired) - just a possibility, and that's why they want a standard plastic fitting installed.