So an example message I'd be OK with is if bojo (boris johnson) had said that the uk gives 160 million (or 199 million[0]) a week to the EU, instead of his claim that it was £350 million a week (which deliberately and dishonestly ignored the rebate), I'd be OK with that.
I fear that tribalism is a disease that's damaging both the UK and the US at the moment; make your point with facts as far as possible.
[0] dependng on how you count, I've heard both figures
And since we typically vote on our shared expectations, "no false claims" would probably mean everybody has to shut up. That might be a good thing, I don't know. I don't believe in the power of elections to bring about change (whatever party wins will have very little room for maneuvering), so maybe removing the loud and divisive parts of them might be helpful. Basically: if they don't help, we can at least make them less harmful.
Do you have an example of a message that you'd be okay with?