Sky Go App (Mac) only downloads in SD?!

I'm trying to watch the new Game of Thrones in the best quality possible. I don't have a Sky Box so am limited to Sky Go and the Sky App on PS4.

Both seem to be giving me crappy quality. Even with an ethernet connected to my mac with download speeds of over 300Mbs, the Mac Sky Go app will only allow me to download in SD. I can't find any setting to change this.

Does anyone have any advice? 

Thanks