With a little help from the Help
After checking out the Help file, the reason for the low score started to become apparent. It was mostly due to the lack of compatible SDTV and HDTV recording hardware within the test the system and not as I'd initially thought because of the relatively dated spec of the system.
At this point it started to become clear that IDHCAT had been designed exclusively for use within Intel’s home market of the USA - without consideration for other markets.
In the UK, the amount of video-capture hardware that uses a dedicated MPEG-2 Codec is very limited. Also, of course, HDTV isn’t yet broadcast over here, so there are very few HDTV video-capture products sold into the UK market.
There are some exceptions – such as the DVB-T-capable card from Nebula Electronics, the DigiTV PCI. Even so, I wouldn't think it likely that that Nebula's card (or any other with similar spec) will work with forthcoming terrestrial HDTV transmissions from the BBC (or any other UK broadcaster) because of differences in the TV channel-space between countries as Nebula itself notes here.
I did mention this localisation problem at the recent Intel briefing and was told to e-mail the development team at Intel so it can be addressed in future versions. But, as it stands, the testing program would appear not to be suitable for assessing the capabilities of PCs built for markets other than the USA.