I would not spend double miles to upgrade on a 40 minute flight. Food is somewhat better and seats wider and with a bit more pitch, but nothing to get excited about. There is a service difference between the LCY-AMS and LHR/LGW-AMS flights, in that in the LCY-AMS flight you only get water and no other drinks in Y (unless they have changed over the past couple of weeks). All in all, I would take the LON airport that's most convenient for you!