Truss and Farage are individuals, and either no longer in power or haven’t actually been anywhere near it. So if they go to the US and appear at a rally, for example, there’s no repercussions in terms of the US-UK relationship after the election, because they have nothing to do with the UK government. No damage done.
The Labour Party sending staff over to help with the Democrat’s election campaign is an entirely different matter, as it inevitably suggests that the ruling UK party/UK government is taking sides in the election and openly preferring one outcome to another.
Depending on the election outcome, that could impact the UK’s relationship with its most important ally, and leave the UK worse off overall. That’s a party political issue impacting the UK, i.e. exactly what Starmer said he wanted to avoid. Once Labour gained power, they should not have allowed their staff to assist in the Democratic campaign, because doing so threatens the UK’s best interests.
Those are the facts of the matter, so I’m afraid a few crybabies on here will just have to deal with that, rather than pissing their pants when people spell it out for them.