Anything using HLS or DASH streaming over the internet (i.e. basically everyone these days) is going to have around 30s delay compared to live, as a starting point, due to the way the video signal is chunked into segments that are then reassembled by the player. The minimum delay is 2x the length of a chunk (which is normally around 10s), but the normal starting delay is between 3x and 4x. Delay will obviously increase whenever the stream is paused for buffering, meaning it's quite possible to be multiple minutes off the live source but still viewed as if live (this becomes an issue if there is a strict cut-off time for the feed, as it means people may miss content at the end of the stream because they have buffered a section of the feed).
DVB Cornwall's stats miss out the important detail of how he's watching each of the feeds, so it's difficult to make comparisons. Satellite feeds obviously incur a significant time penalty from being sent 22,000 miles away from the planet and back again.
I'd need to check a map, but I'm pretty sure Cornwall is less than 22,000 miles from London, as the crow flies.
This is true, but there were all sorts of reliability issues with using crows to deliver live television (dropped packets, buffering in the wind, border overspill etc.) so using satellites is now one of the preferred options.