I think it was predominantly an editorial failure. These types of comments get said a lot during live shows and live recordings, and it's up to the editorial team to decide if it's suitable for broadcast or not. They made a mistake and let it go through. I wouldn't have because I don't think it was suitable for broadcast, especially at tea time on Radio 4 (and I'm probably one of the few people on this forum that listened to the show as it went out).
That said, it is slightly ludicrous to think that a somewhat obscure comedienne in her fifties would have the power and influence to inspire people to go and chuck battery acid in Nigel Farage's face.
A further irony is that Farage himself is a brazen populist who has happily whipped up the masses in hatred against foreigners and anybody who isn't British, helping to foster an atmosphere in which it's more acceptable to abuse people who are different. So maybe he and his ilk need to tone down their rabble-rousing rhetoric too.