I'd love a fan in my upstairs bathroom but getting a neat 100mm hole through the wall is a bigger problem than the wiring...there is a porch underneath which probably means the use of scaffolding for whoever makes the hole. It might be possible to drill it out from the inside with a diamond core bit without spalling the external bricks - if whoever did it was really careful not to exert much pressure for the last half inch. I reckon it would be a £1000 job by the time the builder and sparky had done their jobs, so I'll keep opening the window 
It can't be that difficult to core drill without significant breakout - I managed it (albeit London stocks - softish bricks probably do help with spalling).
More than welcome to borrow the drill, but you'd have to step up to a six inch fan ... I was fitting a heat recovery one.
