We all know that, where Sky have an SD and HD channel, they purposely reduce the bandwidth on the SD one

HD broadcasts are clearly better, although like DVD upscalers, some TVs do a reasonable job of upscaling SD. My MCE does an A1 job upscaling (likes face it, it certainly has the processing power), and does make it difficult to tell SD and HD apart, but clearly there is more detail in the HD broadcast. 1080p films are better again.
But the only things, beyond action films, I want to see in HD are the Natural History programmes, and the BBC make the best. And the 2 BBC HD channels are free to air anyway.