The only job adverts I've seen that ask for a minimum grade at GCSE or A-Level are those that don't require you to go to university... but to be fair I've never done any research or looked for jobs that have anything to do with astrophysics but still strikes me as odd that they'd ask that...
Any logical person would see that you, with a first in astrophysics and a UCL masters, could probably achieve an A* on A-Level Maths even when drunk, but job hunting is anything but logical so I don't know. What type of jobs are you specifically looking for?