I agree its harmless.
I'd hope any investigation was only paper talk as the truth is that there is nothing unlawfully said to discriminate in the banner.
There's no discrimination portrayed for Race, Religion, ***, Disability, Age etc.
The only thing they could possibly site is that the banner needed permission in the first instance, not anything to do with what was written on it.
P.S. Its shocking to get in effect a written warning for flicking the bird, if that's all it was. You could also question if you are actually breaking any laws by doing so.