This is a bit of a specious question.
Firstly, because some quarters say that anything that they don't like is "left wing radicalism". I've seen people describe sections of the Tory party as left wing radicals. And the Current Affairs department of the BBC. And The National Trust. And the Archbishop of Canterbury. Just because some nutty person says something, it's not evidence that it's a consensus.
Secondly, they're not. Plenty of "UK Jews" criticise Netanyahu without being described as such. As you'd probably expect, given he's massively unpopular in Israel. It's a pretty mainstream position to take.
If you're referring (as I think you are) to people describing Jewish Voice For Labour (and those similar organisations who take an explicitly anti-Zionism stance) as left-wing radicals, it's because they as an organisation position themselves on the left of the UK's principle centre-left political party. So it's not exactly a slur, is it?