Why does foreign / overseas programming look so poor if HD is HD???

Just because the re-broadcasting station is sending you HD signal doesn't mean the original programming was recorded on HD.

^^^the station rebroadcasting the programs you mentioned must be purchasing SD and then just upconverting it to HD. I have the same complaint with most the Britcoms broadcast by my local "HD" PBS channel. They are new enough to have been shot in a HD format(Regie Perrin(new), Outnumbered, Worst Week, Last of the Summer wine, etc.) and they are all broadcast in "HD" but definitely not HD and they are even 4:3
I also have a complaint with my most recent DVD purchase, Hetty Wainthropp Investigates Complete Collection. The quality looks more like VHS than even DVD apparently it wasn't remastered from the original source! good material though.

No, as many parts of the world have a 50Hz electrical system, which means a 50Hz to 60Hz conversion is necessary that can also cause undesirable image quality issues.

There are also a number of other variables, including how the video gets from point 'a' to point 'b'.

From Europe, many broadcasts are native 16:9, but not native HD.

Blame the local stations

50-60Hz conversions can be anything from amazing to complete crap depending on how much money they want to spend. The World Cup matches on ESPN HD last year were almost indistinguishable from native 60Hz, while a lot of National Geographic shows fall into the "complete crap" category with horrible blending in the video and bad blocking because the blended frames ruin the compressibility of the video.

The most recent Euro production was the Royal wedding(1080/25i) which looked indistinguishable from our 1080i/30, so theirs no reason it can't look very good, just probably costs more than smaller productions are willing to spend.

Here's the oddest thing. I have British DVDs that are "Region Code: PAL 2" and contain 50i video material that was not from a film source nor were they "flickered" to look like they were filmed. I play these DVDs on my HTPC connected to a 1080p 60hz HDTV using free software... and the content looks great. I'm totally unable to see any artifacts of watching 50i on my 60p display except maybe a little judder during panning.

It makes me wonder if 50-60hz conversions are really that complicated. The software is not doing anything special to make this happen. My best guess is that since the deinterlacing is happening on the PC, I'm actually seeing deinterlaced 50i at 50 frames per second and that can't happen in broadcasting since the content must be interlaced again.

...And many conversions from PAL are rubbish.