Yep. You *can install a velux window working totally from the inside loft space. I was working with a retired roofer on my own house, we put the window into the back facing roof of a terrace, I.e. not visible from the road. Being bold as brass, matey decided to climb out onto the roof and do some of the fixing and tile cutting from the outside. Obvs need a harness or scaffold for this sort of thing. The work in its self isn't all that complex. It just takes time (half a day or so) and you need to be meticulous but it's totally achievable. It's so nice to just stand out there at night, head popping out of the roof.
Eta. Velux brand roof windows are the absolute standard. Don't compromise. They're rock solid.