What I would expect a worsening economic situation to do, and there's plenty of evidence that it will do, is discredit governing parties. AFAIK since 2008 no incumbent EU government has won an election. In some countries that's already leading to alarming drops in support for both sides of the traditional political elite. Now, it's not a given that such a thing will convert into increased support for the Far Left. But if it didn't it would make the UK abnormal in that respect. Now, whether such a shift would be of any use to anyone is debatable.
The question that's been asked, however, is what does 'the left' have to put in place of capitalism. It may be true that in some countries the establishment parties will be discredited enough that more radical parties will be propelled into government, but they'll merely be in office and not in power, trying to run capitalism for the unaccountable and untouchable oligarchs who really run the show, and trying to satisfy a bitterly divided and largely hostile population.
This applies equally to the far-right.