Not quite sure the two offences are comparable.
While I have no time for the group the flag represents, it would be deemed as freedom of expression, but in poor taste.

Wrapping bacon around door handles of a mosque would be seen as racially motivated hatred, so taken more seriously as it is a physical act against a group or faith.

I understand the point you are making, just don't think there are grounds for arrest unless the flag waving was accompanied by shouting racial hatred, or preaching in favour of the groups actions.