The International Space Station will be passing right over London next Thursday (22nd July) and will be both exceptionally bright and at a decent time to see it.
The flypast is just after 9pm coming from the west.
Once you used to the twilight watch for a bright star that rises up over the horizon coming directly towards you. It may be tricky to spot initially, but once you see it, it will jump out as something very noticeable in the sky.
The further away from street light the better, twilight in a back garden or above street light level has the best chance.