Come on Foggy! Is it thick tonight? :P
Firstly - distance - Cardiff is further away from CDG/AMS than BHX - that's the first obvious thing to point out.
Additionally, traveling to BHX for the sake of £75 - it's a heck of a distance to be picky. The difference is, KL can fill 73x's out of BHX, but it can barely fill the more expensive F70's out of CWL - so naturally the cost of you being on a half-empty F70 from CWL for £37.50 each way extra - it's to be expected.