Same as up to xxx MP3s
Bitrate may not be as important as the codec used, but it's still a major factor.
I'd query where you get your figures from on DVR recording bitrates.
H264 1080i HD broadcasts from the BBC over Freesat average about 3GB an hour when recorded to my PC - or about the same as an SD MPEG2 from a DVD. Given how much room HD content takes on my SkyHD box I'd be surprised if they are re-encoding it to reduce the size before storing it. It adds a lot of complexity and adds an extra point of failure to what should be a fairly simple process (take the transport stream and store it directly).
By comparison, my Blu-ray of Watchmen (also H264, but now 1080p) is 38GB for a film of about 2.5 hours, or around 15 GB an hour. Admittedly this is an extreme, other rips are around 25 GB per film.
Given this I calculate I'd manage 160 HD films at BD quality (4000/25), or 800 HD films at broadcast quality (4000/5). Obviously these are rough figures and ignore the 1024/1000 rounding that disk manufacturers get away with.
I personally would argue that anything supposedly in HD that is at less then broadcast quality is bordering on being no better than scaled good quality SD. All that real estate demands a decent bitrate, and squeezing a HD film into 2 GB (or in fact significantly less when you factor in a 384 kbps 5.1 soundtrack) does not sound like quality to me.