Freedom of association means that businesses can generally serve who they want. Declining to serve spammers is not "interfering in foreign commerce". Declining to serve abusive jerks is not "interfering in foreign elections".

Freedom of speech is not a right to a platform. Freedom of speech also isn't the freedom of consequences from one's speech. I'm free to say what I want. Other people are free to, based on that speech, decide I'm an asshole and that they don't want anything to do with me.

You just shifted the goalposts on a massive scale. You weren't talking about "abusive jerks" were you? You were talking about actual British citizens who ran for election, which makes them politicians. Whether you like them or not is irrelevant - if "abusive jerk" was the bar, no politician at all could use social media, as there'd be a significant number of people who felt that way for all of them.

In practice I can see that you're fully in support of censoring politicians you disagree with, whilst allowing those you support free reign. That's an exceptionally dangerous path. You certainly cannot hold that view and also make arguments about "foreign interference in elections" being bad - beyond the total absence of proof around what you're thinking of (Russia), Facebook blocking local politicians is by any definition a foreign company interfering in local elections. The fact that you support the move because it's against people you dislike is irrelevant for avoiding a charge of hypocrisy.