ITV4.
You can watch it online on ITV4 too, though the ITV live player is shocking. I'm going to take this opportunity to have a rant at ITV. They haven't got anywhere near enough bandwidth for the live streaming, so if you're watching anything at a peak time it's frequently buffering and there's no rewinding like there is on the live BBC iPlayer (where if you've missed the start of the F1 qualifying because you fancied a lie in, as I did today, you can just rewind to the start of the programme when you do stumble out of bed).
I wouldn't expect the rewinding feature iPlayer has (this is ITV after all), but I've been infuriated with the buffering on ITVs streams, and yes, I've got a very stable 8mb net connection, and can happily stream 1080p, nevermind what ITV use (which isn't HD of any description).
For the record, I normally wouldn't bother with BBC or ITV streaming for live TV, but for some reason, since the digital switchover last autumn, I've had sod all reception. I've been using digital perfectly fine for years now, but for some reason when they flicked the switch to turn off the analogue signals, my digital reception fell through the floor. I need to look into that at some point.
I might give the browser version of TV Catchup a go for the next BTCC meeting and the semi-finals of the Champions League and see if they're better at streaming ITV than ITV are at streaming ITV... they can't be any worse.