Hmmm...although I do neither, I'd imagine Economics or Psychology to be useful to the average Joe. Possibly even History. The economy is what everyone bases their votes on and psychology as you're meeting people all the time and knowing the (possible) meanings behind how they act could prove beneficial.
The sciences have given me a profound appreciation of the world, but it's certainly not "useful" in my day to day life. And for all the people saying Maths, really? As fun as it is, how is anything in the A-Level syllabus relevant to life? As far as I can tell, the only part of maths useful to a random person is Arithmetic, and you won't find much of that in A-Levels.