The BT Sport Boxoffice app is a strange one, bit like Sky's, where you'd think big screen platforms would be as widely supported as the other Sport app's are (BT Sport & Now TV ect) but no, its not.
BT Sport Boxoffice does allow Airplay, Chromecast and watching on the Amazon Silk browser (So you can watch it on a Fire Stick & Cube) but the lack of sibling releases such as XBOX ect is frustrating, because for those of us that aren't tied to a TV provider, you're seen as a bit of an outlier when it comes to something you'd think would be a priority: Pay-Per-View!!
To watch Sky Sports Box Office, i have to hook my laptop into the TV with HDMI and use the desktop SBO app, its garbage but the only option i have, despite Sky adding Chromecast support to the SBO app, because they then disabled it despite it being there in the code. No Airplay and blocked from Silk too, so you can't watch it on almost any big screen unless you like alot of cables and faffing about.
Go to the US, and you can watch almost anything how ever you like... PPV included, same in Australia too, but apparently, mobile phones is where its at in the UK !!