At the latitude of London the sun clears the horizon for seven hours and twenty minutes, and the rest of the day it does not. The word solstice means standing still — the sun appears to stop moving south, turns, and comes back.
From sunrise to sunset on the shortest day. In June it is sixteen and a half.
Long enough to walk a long way and still be walking when it ends.
The solstice is not the coldest day, and it is not the shortest. Both of those come three weeks later.
Day length depends on two things: how far the axis is tilted, and how high the sun climbs. At the solstice the tilt is at its maximum and points away from you, so the sun traces a short arc across the southern sky and spends most of its time below the horizon.
The chart above is the declination curve, which is the sun's height at noon as the year turns. Notice how flat it is near the solstices: that flatness is the whole reason the change in day length is abrupt rather than gradual, and why the days seem to stand still in early December and then suddenly begin to lengthen.