Indycar had a very settled and long term home on Sky and switching channels, include a name change 6 months in, has knocked it somewhat but the good think to take into 2015 is that BT Sport and the producers are very keen on motor sport and developing their coverage. The non 500 races used to be around 15-20,000 and are on the up from 2013.
Here's the disclaimer, I do not work within any TV organisation, data collecting or anything other than plonked on the sofa on a Saturday or Sunday evening, how accurate these figures are is sometimes debated, but that's what is out there!
Worthy side note - These figures are for live coverage only. Sometimes at unsociable hours, up against competition, like Downtown or something, looking back at some of the highlights ratings, Milwaukee pulled a 6 live, but 13 highlights the day after.
Every month during the race season, I would estimate around 80-85% of the search terms that bring people to this site are about Indycar coverage in the UK, how, when, where and more.
So for those finding this blog by asking... Yep, Indycar is on BT Sport 1 / BT Sport 2 / ESPN this year (2015). I know the possibility of growing the coverage here is on their radar, but can't promise anything, so to ensure Indycar stays live on UK TV, each race day just tweet about it, it helps!
Also good promotion helps, twitter, facebook, actual advertising on the channel, there is no coincidence that the Indy 500 was as high and better than when on Sky Sports after BT Sport aired their own commercial in the weeks prior to the race.
A good blog to follow is F1 Broadcasting, if TV news and stats are your thing, mainly concentrates on F1 and Moto GP but does cover other series from time to time.
Moto GP - Live on BT Sport.