Bernard posted a diagram in another thread.
As can be seen in the diagram the fuse *does* connect directly to one of the poles on the isolator.
I suggest implementing this by using the isolator/fuse plate as the junction box. The light, switch and fan can then be connected to the isolator/fuse plate using normal T&E and 3+E cables.
Of course, on this diagram, the fuse protects the bathroom light as well.
There was a diagram on another thread where the 3amp fuse ONLY protects the fan (both perm live and sw live).