Ok then compared to freeview sd the picture is worse
From what I've read of the early Now TV box it outputs at 60Hz instead of 50Hz, which causes issues with many TV's in the UK. It appears some are better than others in handling the resultant choppy pic quality. As I understand it the newer Now TV Smart box outputs at 50Hz so would provide a steadier picture. Either way 720p should provide a sharper pic than SD.
If you can't see some sort of improvement between 720p and sd there is definitely an issue with your setup. Even a good 4k tv will give more than decent results with 720p.
Indeed the newer Now TV Smart boxes are much better at handling motion issues etc than their predecessor for the very reason @Hunter2660 points out, the old boxes output at 60Hz with the newer one outputting a much more UK friendly 50Hz.
This is a reminder that the BT community forums are intended for the discussion of BT products and services of which Now TV doesn't fall into so I would ask that you please do not discuss it's features or pricing here.