I think there's a fairly fundamental difference between inciting a riot by saying 'Hey, let's all riot' and inciting one by insulting someone enough that they feel the need to kick off.

The first one is already illegal in the UK as it's incitement to commit a crime - which is a crime in and of itself.

The second one is more of a grey area and the question really should be: Should you be allowed to get away with insulting someone or their beliefs just to boost your circulation?