Doing a weather check at around 0615 Sunday morning, I was surprised to
see a distant flash of lightning.

A look at radar returns suggests that the storm was in the Southampton
/ Winchester area some 200 kilometres from where I was observing in the
south of Cambridgeshire. I am impressed by the distance that the storm
was visible. However, a calculation of line of sight from my vantage
point at over 400 feet above sea level to an object (flash) at say
25,000 feet is in excess of 350 kilometres. It was just a slight
surprise to realise that distant flashes can be as far away as this. I
had tended to assume (without really thinking about it) that they were
much closer.

Jack