They say they want to make their voices heard, not that they want to disrupt the debate, but however you interpret their wishes, take their plans and Facebook announcements with a big pinch of salt. Neither EDL, nor the shouty 'smashers' of the Social Workers and the UAF are going to be tolerated if there is seen to be any danger to the three party leaders or to the TV show.
Heckling used to be an almost universally accepted part of public political meetings, but not now, I'm sorry to say. Nowadays, old men can get thrown out of the Labour Party conference for audibly dissenting.
I don't know how people get tickets and are allowed in to the leaders' debates, but the audience for the first one seemed painfully well-behaved. I see no reason to think it will be different for the next debate. The chances of many EDLers or their 'smashers' sneaking in are low.