Europe is very small and most flights exCDG to any major European points require one hour in the air (or less). Exception could be ATH, but that is a minor destination. Whether you fly from HND to LHR, CDG or FRA (the three major European airports) makes little difference if you need to fly to another European airport. Unless, of course, you end your travel in LHR, CDG or FRA. LHR is very constrained in terms of slots. There is no Japanese airline in ST, so AF is "neutral". Codesharing with a *A partner like LH would be difficult.