I've been giving 'Robin Hood' and 'Merlin' a run out now they're available for this twelve months. The HD upscale on both, if it is upscaled and not acquired HD from somewhere, which I really doubt, is astonishingly good. Some work has gone into the release of both series. I do realise that the final Merlin series is native HD btw.
I always thought Robin Hood was shot in native HD as it wasn't as VFX heavy compared to Doctor Who and Merlin. Bleak House IIRC was one of the first BBC drama to shoot in HD.
Wikipedia suggests Robin Hood was shot at 720p. The first series was one of BBC’s first HD releases on home media - so early that it was one of the eight to get a HD-DVD as well as Blu-ray.
Merlin got all of it’s series released on BD stateside. They’ve only recently started to upscale SD content on the format (Doctor Who, Hitchhikers, Red Dwarf, Fawlty Towers), so we can be fairy confident that Merlin is at least 720p too.