Dave,
First, he is talking about a widower whose wife

No he was not! He was talking about a political organisation.

Second, he smeared an organisation that exists to drive back racism

It is just another political organisation, not some Crusaders for Righteousness! Of course it is OK to criticise them and challenge their methods. If he is right about their use of violence, it is no smear.

inflammatory by politicians including Nigel Farage.

Jim,
how Ukip is generally regarded, not just by the left, but by the country as a whole

London is not like the rest of the country.
It voted strongly to remain.
It elected a Labour Mayor.

Its main local broadcaster is LBC.
Would they appoint a presenter its audience would see as loathsome or repulsive?
Of course not, but they have just appointed Farage as presenter.
But you can not quote a single inflammatory statement from him.