I'm sure you're aware that AppleTV boxes and NowTV boxes run on wholly different infrastructures.
NowTV on AppleTV is purely the NowTV app, only some of the additional apps available on the black or white NowTV boxes are replicated via the AppleTV App Store.
Channel 4 All4 and ITV hub catchup is NOT currently available from the Apple App store and therefore aren't available via AppleTV. The BBC iPlayer was only offered in mid December. You should perhaps contact Channel 4 and ITV to ask why they're not offered.