You’re not going to like this answer!
UPC simply provide the options given to them by the channel showing the programme. In this case, RTS Un have it set as two languages (French and English) probably for every programme they transmit, but for shows with no English they simply put French on both audio streams.
There’s nothing UPC can do about it. You’d need to get RTS to transmit English audio. As GoT obviously comes with English they’re just being lazy by not doing so - unless there’s some kind of licencing issue I suppose.