Visible passes
The ISS over London
No visible passes over London in the next five days — the station is going over, but never while the sky here is dark.
The station is bright enough to be unmistakable — brighter than any star, moving steadily, never blinking. It crosses London 6.2 times a day on average, but you can only see it when three things line up at once: it is above your horizon, it is still in sunlight up at 420 km, and the sky down here has already gone dark. That is why sightings cluster in the couple of hours after dusk and before dawn.
Over the next five days the station passes over London 31 times, and none of them are visible: every pass falls in daylight, or the station is in Earth's shadow while it is overhead. This happens for a week or two at a time and then reverses.
How to watch
No equipment. Be outside a couple of minutes early, look towards the direction given above, and wait for a steady white point to climb out of the horizon. It moves about as fast as a high aircraft but shows no flashing lights and makes no sound. A pass takes a few minutes from edge to edge.
Passes below 10° are left out: at that height the station is behind whatever buildings and trees you actually have, and there is enough air in the way to dim it well below the brightness quoted here.
Computed with SGP4 from orbital elements issued 11 hours ago by CelesTrak, refreshed every hour. Element sets decay, and the station manoeuvres — times more than a few days out will shift. Also see what else is visible tonight from London.