slightly different.
For what it's worth I would still have done the cu upgrade

What a surprise

The way I read the OP the issue is that the glaziers have installed a fan which is supplied prewired with a flex which the tiler and/ or others have obligingly set into the wall then tiled over it.

So if it is prewired with a flex by the manufacturer it will only have the required cores within it. So if the fan is double insulated it will have two cores live and neutral, if it is class I it will have three cores live, neutral and CPC, either way it is not going to have a spare core, be it a CPC or otherwise, to be utilised as a switch live.

So any discussion on the subject of using the CPC as a switched live is totally irrelevant.

Therefore with that option crossed off the list of remedies you had all better put your thinking heads back on

Andy

Your right Andy. But we don't have to put our thinking caps back on because no action is required. No overrun is required.