If I understand it correctly:
given 2 bodies travelling at different relative speeds, time will appear to run faster for the faster moving of the two, from the slower objects perspective
a single light photon will be seen to be moving at the same speed for both because although the 'distance travelled by light' value will be greater for the faster body, the 'time it took to travel' value will also be greater, resulting in the same speed measurement (x/y = 2x/2y)
However, if both bodies are moving in opposite directions, as we have no fixed point of reference, who is to say which is travelling faster?
(The next part of the question was about being able to work out the rate of expansion of the universe based on a "fixed" point relative to light, but I'm not actually sure how to translate that from my brain into words! I may come back to that...)