Because the information density of HD television is higher the signals interact with the atmosphere rather more. You can visualise it as the signal having a rougher 'surface' (because there are more bits sticking out). The effect of this is to delay the speed that the signal travels through the air. This is what we scientists call 'propagation delay'.
So it might be 21 seconds where you are Richard, but for other viewers it could be much more or much less.
The effect is much stronger in areas where the air is polluted, so in Barnsley programmes can arrive several days late. Unfortunately this effect causes (as Einstein predicted) a 'time dilation'effect, so people living on the other side of Barnsley (for example) receive signals that are bent around the Barnsley zone. Since the signals can go around Barnsley both ways (the new one-way system has no relevance) these viewers receive their signals twice. This explains all the +1 channels.
Obviously satellite signals have to through a lot of atmosphere since they are very high up (probably ten miles or more), and this explains why the men in Top Gear on the satellite channels always have 1970s haircuts and are testing Ford Cortinas.
There is a related problem called 'phase delay' in which the top of the picture is delayed more than the bottom. This is why people on Eastenders sometimes don't seem to know whether they're coming or going. Luckily the BBC Engineering Dept have found a complete solution to phase delay, involving a relatively simple alteration to the way television programmes are made. At first there were problems with the actors' uniion but these have been settled. Actors are paid to come and paid to go irrespective of which they do first, but where they are called upon to come and go simultaneously they get time and a half.
Incidentally I just looked this up on Wikipaedia and found nothing. That's a gap in human knowledge that needs to be filled, so I'll be expanding the material I've covered here and making an entry. Can anyone else contribute to this as I feel that my knowledge, although extensive, is incomplete.
Bill