Just reading this article and it would appear that it's only 40 pound more to get a Fenix 5x over a a Fenix 5 premium with sapphire screen. If you get the base Fenix 5 then it does not come with sapphire screen or wifi or maps. But if you get the sapphire version the difference in getting a Fenix 5x with maps and navigation is only 40GBP
http://www.activejr.com/garmin-fenix-5x-vs-fenix-5-feature-review-maps-battery-design-watch-best/
Or this excellent video comparison
https://youtu.be/9E7ptHgmpXI