LHR - Not been to the new lounge but I'm guessing its very good. It needs to be because T3 is in general a hellhole.
LGW - Check in staff are good (the manager is from LHR). I like LGW because of the A350 and the short taxi but it can be very busy during holiday periods.
CDG - Horrible airport in general. The lounge is good but not great.
AMS - Rubbish BA lounge with great views and must be getting close to a mile walk to the usual CX departure gate. The airport overall is pretty good though and a nice place to fly from.
FRA - Fairly standard. I've only been once.
DUS - Never been.
MAN - The whole place is geared up for low cost carriers. I like it because it's easy to get through quickly but don't spend much time in there.
MXP - Pretty basic contract lounge and not much else to say about it.
FCO - The BA lounge is big but was last decorated a few centuries ago. Otherwise pretty standard like MXP.
ZRH - Never flown CX out of here but it's usually a pretty good place to fly from. Clean and efficient.
MAD - Not been to.


All in all i'd go to the one that's most convenient. I don't know why you'd fly across a continent to get a better airport experience.