I guess it technically is HD, as you can upscale it to HD resolutions to take advantage of higher bitrates & 50fps playback. Though obviously it is still just upscaled SD.
Although thinking about it, wasn't the House Party one of the shows that was recorded in a type of HD as part of a BBC trial? Quite exciting if we get to see what that looked like.
It was one of the programmes they trialled 14:9 on to gauge public opinion on having black bars on programmes. I think they did a whole Saturday night, maybe a few.
I doubt it was shot in 16:9, they presumably just put black bars on the output