I have a good view of the approach to LCY
from my desk here, and I haven't noticed a white F50 going in and out on any regular basis - but then perhaps I haven't been looking out the window at the right time.
Just a guess, but I wonder if it's a recent VLM aquisition? When I last flew with them (maybe six weeks ago) the in-flight mag said they were hoping to add more F50s to their collection (so far, they're a one-type airline). LCY
is pretty much their hub, so if they were going to park it somewhere, City makes sense. They're already flying at least one ex-Denim F50 - PH
suggests it was flying for KLM Cityhopper as recently as October last year though, and was all-white at that time. Not sure why KLM wouldn't park it at AMS