Sometimes fan manufacturers require you to put the fan on a fused supply. This isn’t always easy, particularly if your fan needs a switched live and a permanent live. If you were of a cynical mind, you might think that they specify a fuse knowing that it often won’t be done so that they can get out of warranty claims, but I couldn’t possibly comment.
However, if you replace your light switch with a double pole one, it is easy to use a fused connection unit in the circuit. And if you use a switched FCU, you will no longer need a 3-pole fan isolator switch, as the FCU will provide complete isolation.
The above plan is a theoretical diagram and not a very practical layout, here is how you would actually wire it (you could also loop in at the fan but i get the feeling that may be a tight squeese and would also probablly require more wiring)
CPC added by empip 240307.