If the peak figures are correct then it shows that hundreds of thousands of people watched BT Sport for the first time during the City v Liverpool game. The peak figure of 1.8m is a lot higher than the previous high, so this shows that lots of people who had access to the channel hadn't actually used it up until Wed. So, that game was a breakthrough game for BT. It got people tuned in for the first time, got people to download the app for the first time and made people aware that they have big games.
This was the bonus of having a big game on Boxing Day. Had the fixtures not been kind to BT and had they ended up with West Ham v Arsenal and (let's say) Man City v Newcastle they would still be in a situation right now where hundreds of thousands of their potential viewer base had still not bothered watching a game on the channel. But, now they have. They now know that the channel has big games, they know that Sky don't have all of the big games, they know that they have the app there now, etc, etc.
This should result in them coming back on New Years Day and in the big midweeks when BT have Liverpool v Everton, Spurs v Man City and especially Arsenal v Man United. I personally didn't think Boxing Day would work out as well for them as it did. It's a day when families dominate the TV set and football sometimes doesn't get a look in. But I was dead wrong, and the kick off time of 5.30 must have helped because it sat between the afternoon TV and the prime time stuff. I also think there may have been many football fans fleeing the living room to watch the game on the app or the website as well. All of it helps BT gain a foothold and awareness.