It's similar to installing an outdoor power outlet: https://www.familyhandyman.com/project/how-to-add-an-outdoor...

My TV aerial is in the attic space, and it works very well (although I do have a line of sight to the transmitter). The cable runs down the wall.

I wonder if a 4G aerial would work as well in this kind of environment? The roof is wooden rafters with a slate covering.

Yup, definitely a head scratcher! So far I've been able to thread through an open window but when the weather turns I'll need to drill a hole somewhere at some point :)