A convoluted argument!
But I can watch, for example, the BBC News live stream on the Internet simultaneously with it being broadcast on conventional TV
No! "The law states that you need to be covered by a TV Licence if you watch or record television programmes, on any device
, as they're being shown on TV
is it simply a matter of trust when I declare that I wont and hence a licence will not be needed?
No, it's a matter of law! Put simply, if you declare you don't watch while it is being broadcast, but you do watch on any device whilst it is being broadcast on TV, you are breaking the law and you take your chance whether you get caught.
On your previous post, if you have a device that is capable
of showing TV progs as they are broadcast but you never use it for such, then you don't need a licence. E.G. I can buy a ordinary TV and use it solely as a PC's monitor or with a game console. then I don't need to buy a licence (if I have no other TV device).
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 19 Meg WBC