Looks closely and see about 6 miles away is Didcot Power station, likely to receive a direct hit if a full scale nuclear exchange where ever to take place.
I'm standing atop the Chilterns at Nuffield. At this distance I would be largely safe if I shielded myself from the initial few seconds of intense light, say behind a car. I would certainly feel the shock wave which would hit about 20+ seconds later travelling at the speed of sound, but could stand against it if I braced myself.
If the burst were made to hit the ground, which would create a lot of fallout and be an unusual military tactic, I'm at 90 degrees to the typically prevailing wind direction which would take the danger toward the right in this photo. If the wind were travelling in my direction, I'd either need to vacate the area quickly and in the fastest way (calculate the best direction to drive fastest from the cloud, much like the Enola Gay, but for her just to escape the blast, probably about 8 o'clock from ground zero)
If I could not escape the fallout cloud in time for some reason I could have prepared in the build up to the attack by building a simple fall out shelter to survive, shielded from the gamma rays by 3 feet of simple earth and with a dog legged entrance so that no gammas could shine inside. After say 3 weeks enough of the settled fallout would have decayed away for my cells to permanently tolerate the remainder. Obviously take more care with the young and pregnant before going back outside.
The standard yield for best destructive effect for a nuclear weapon is 100 kilo tonnes. This would be detonated just high enough for the fireball to not quite touch the ground(about the height of the cloud base here) causing a 'mach stem' shock wave across the plain - this effect intensifies the shock by reflecting the blast wave thats already hit the ground onto the direct shock wave. Thus destroying by the total blast as much as possible in its wake.
But I expect the bomb wont be detonated too low that large amounts of fallout are created and sucked up into the lower atmosphere. If it was it would spread downwind, north-east in the UK and then slowly fall to the ground over maybe a narrow 50 mile cone. A 100kt bombs best detonation height is likely to raise the fallout cloud so high into the stratosphere that the dangerous effects of fallout will be rendered largely infinitesimal, most of it maybe never making it to the ground at dangerous dose levels.
Then it would be a matter of using wisdom followed by intelligence to find food and water and to start the inevitable recovery with other sensible people. It all goes in cycles.
Knowledge, is Power.