With respect to the UK, in a massive nuclear attack, defining 'the middle of nowhere' is a tad tricky. Not to mention being outside is not recommended due to fallout.
Somewhat agreed, then not agreed at all. People forget that nations are big places in terms of area, and hardly ever look at totallying up a number of 'blast zones' and doing a ratio with comparison to the nations size.
Lets assume we drop N weapons, of 20Mt each, each bomb has a blast radius of 13mi where we can consider 'widespread damage' to occur. This is not total devistation, I am over estimating blast effects
by a large factor for the sake of illustration.
We have 530sqmi per bomb of terrain affected. The UK has a size of 94,060sqmi
So thus you have a % of affected UK nation formula of;
% = 0.56N
For the record; 20Mt weapons are not going to be raining down in massive numbers. But one could look at this as a 'worst case senario'.
However it also allows us to calculate the likely probability for 'random survival' (not a good idea to assume randomness however since weapons are targeted). Which would be;
(1-N/177)^N
Plug in what numbers you want; If I take 20nukes, that's a 9% Survival chance if I 'don't run' and just stay still. Of course nukes being targeted means this isn't the forgone conclusion, running to the middle of the Yorkshire Dales would very much increase your chances of survival.
Fallout is no worry at all for anybody with a modicum of protection. Even a paper bag will save you from contact with fallout.
Therefore a truck in the middle of the countryside, so long as its not got a fan pumping air from the outside in, will be perfectly enough to protect you.
If you want, I'll even calculate the fraction of radiation that can make it inside a camper van if it will make you happy. Fallout is by far the least of anybodies concerns so long as your considered it and planned for it. It's just unfortunate that people have be predisposed by the media to fear radiation like the boogey man in the cupboard.