This does seem a rather strict enforcement of a 'political' group. I could understand that the IRA or Al Queda might reasonably be refused but the Forest Hill Society?? I sense some deeper underlying history here in truth.

I note that you were still able to do some 'politicking' within the park. This got me wondering what would happen if you just set up a little table in a corner and put up a sign saying 'Forest Hill Society'. It is after all a public space and I don't really see that the Traders Association or anyone else has a right to transform it into something else.

I presume that permission is given by the Horniman Trustees to run the event. Did they have any say in the decision to exclude?