Which, if they did this,would ruin my plan, and I may as well have just purchased a 500GBP oneway ticket in the first place.The issue is the OP actually wants a refund for the return segment of a single e-ticket, so the issue isn't being charged extra, but rather, refunding the return segment. In this case, I'm confident BA will recalculate the entire ticket based on what was flown (a one way LHR to CDG) and use this as a the base cost for calculating the refund.
Google, and this forum, don't seem convinced either way.