There would be interference if it came to it, but there are indeed so many ways it could be derailed before reaching that point. I mentioned the US because I have no doubt they'd take a keen interest, but there wouldn't be any need for them to get involved directly when there are so many forces native to this land who would be up for the task first. It would be a last resort if people across a broader range of society were swinging well to the left in a way that threatened all sorts of US interests.
I don't really think that game has even begun, not when it comes to the likes of Corbyn taking power. Because at this stage the threat is not of him getting into power, its that the 'spectrum of legitimate debate' as tightly defined by the media and political classes might be widened, against the flow of the last 36ish years.