Prewire for what exactly?

Either 2x https://www.gowifi.co.nz/cabling-coax-pigtails/p-39.html 

or 2x https://www.gowifi.co.nz/cabling-coax-pigtails/p-41.html 

Fat end on the roof where the antenna's will be mounted. Small end where you want the router. Make sure the outside end is well wrapped to stop moisture getting.

Be aware though there are usual Health n Safety stuff for the installer to consider - so this could determine where installer can actually mount the antenna. I.E. do you have high pitch roof, is it more than single story, is it brick and tile???

ok thanks for that.