Fly out from LHR-AMS. If you have lounge access by having (or your travelling companion having) a Silver/Gold card, then go in ET. If not, spend the extra miles to go in CE to get lounge access.
For the return pick whichever route is most convenient, bearing in mind that the AMS-LCY route in ET has minimal service (a bag is chucked at you, containing a nasty biscuit, a mini bag of nasty savoury bites, and a cuplet of water - enough for one gulp). No other drinks, no coke, no coffee. This is ET service at its worst.