But the problem is that films on BBC HD are broadcast in 1080i50. That is not going to be converted to 24p by the TV. The only way to have 24p is by a 24p source like blu ray.
No it will be converted to 25p. As a result you will get the quality of Blu Ray (except in fast action scenes) and 4% shorter film
Think of it like this.
To make a blue ray, you store a series of images each 1980x1080 at the rate of 24 a second. When you play them back you put them on the screen one after another at 24 a second.
For a movie transmitted by BBC HD, they take that same stored series of images (1980x1080 at 24 a second), and transmit them as follows
Frame 1 They Transmit all the odd lines of Image 1
Frame 2 They Transmit all the even lines of Image 1
Frame 3 They Transmit all the odd lines of Image 2
Frame 4 They Transmit all the even lines of Image 2
and so on.
But they transmit at 50 frames a second.
In the TV at the receiving end with proper pulldown, :
It receives frame 1 - and sticks all the odd lines of image 1 in a buffer
It recieves frame 2 - and sticks al the even lines of image 1 in a buffer
it then displays the buffer for a 25th of a second
It receives frame 3 - and sticks all the odd lines of image 2 in a buffer
It recieves frame 4 - and sticks al the even lines of image 2 in a buffer
it then displays the buffer for a 25th of a second
and so on
The end result is the same images displayed, but at a slighlty higher speed (25/24ths')
Now because the encoding isn't really a series of pictures, but the differences between them (though the end result is the same), Blu Ray has the advantage in that it can encode more information about the changes as it has more data space (or bandwidth) to do so. But for scenes with little or no motion there is no difference.
James