The other issue to consider for a longer layover is lounge access as the CDG-LHR would be origination so subject to access of a max of 3 hrs before departure?

Was looking into award tickets a couple of months back and noticed that while tickets to/from LON (and other southern UK points) were available, they were considerably more expensive in miles - at least 2x miles - as well as UK APD - than tickets to/from CDG and ZYR, for the same date TATL flights which I thought would be the determining mileage cost factor.