UFO…but not necessarily alien

This could be posted several places……

Looking South from Southampton UK at 17:50 tonight.
Large white light travelling from West to East. No strobes at all. It was too fast for a reflecting satellite or the ISS, and far too slow for a shooting star.
The light was about the intensity of a light A/c landing light.
There was no sound, so I think I can rule out the Police copters.
As it passed, the white light went out, there was a red light at the rear……and I know it’s a cliche but it accelerated away about twice the speed it was doing when the white light was on.
I thought all airborne machines had to have the strobe at all times, is that not so?
If it is so, then what did I see?