The data has to be available for StDef and HiDef alternatives to be offered, and it is only BBC1 that does this. When BBC HD becomes BBC2 HD we might get it there too, but who knows why ITV and C4 don't support it.
The problem with the Humax keyword search is it has a 400 item limit, so if it finds 400 occurrences of the word 'Jewellery' from the Jewellery channel before it finds that movie you were looking for, called 'Jumping Jewellery', it won't be listed. The same is true of the Genre search, there are always 400 things with the same genre label that you don't want that prevents the things you do want from being listed
Well this is only returning a page or two of results but can still miss the HD stuff. I think it's possible that the EPG hasn't populated for whatever reason and has managed to fill itself by the time I get to actual show in the EPG.