I would lose the flag.
I like the flag. Plus the flag is there for safety. Especially at roundabouts, cars often only just see me at the last minute and even then attempt to enter the roundabout as close to me as possible. it could be the case that they don't even notice the trailer, so the flag has to be there to warn them i am towing something. just like when builders etc have to tie something, often one of the fluorescent jackets to ladders which protrude beyond the length of their vehicle.
Keep the flag!
i want to add flags from each country I cycle through to it :)