See here, it might show a little more info.
http://www.bbc.co.uk/bbchd/
I know they used to show both, though I think all programmes are upscaled to 1080i even if SD content. Usually the evening programmes are all HD though, especially the big named programmes such as Bleak House, Robin Hood etc etc.
I will ask a few people here as they speak regularly with the BBC about this service and know a lot more. But, I had heard that they were showing more HD content all the time, especially at peak times.
They used to have a schedule guide that told you if the programme was HD or upscaled SD!!
Remember though, peak time programmes on standard BBC1 in SD will have higher bitrates anyway, so the leap to HD will not be so high!
Though you will still notice the difference.