As long as the aerial can get a decent signal the location doesn't matter, roof or loft will both be fine.
The half price aerial install is only when adding BTTV, after i don't think you can get the discount, worth asking though.
An aerial is an aerial, it just has to pick up the siganl, nothing fancy needed.
Youview boxes require an aerial and broadband connection for each box in use, wired either by an ethernet cable or using broadband connectors, also known as powerline adapters.
Unlikely, unless its one that is a normal one intended for fitting in a loft. You need a good signal, so unless you are within a couple of miles of the transmitter, an indoor aerial is unlikely to work.