Policy detail was not put to the vote, and not agreed in advance of the vote, and to pretend otherwise for the sake of advocating a particular approach to withdrawal and post-EU settlement is simply dishonest.
Agreed, but things like that attempted Lords amendment to legally require the govt to guarantee the rights of UK expats living in Europe - something not within the govt's power, and therefore sending them to a gunfight with a wooden spoon at the negotiating table - was equally dishonest. I hope you won't dispute that some attempts genuinely are trying to overturn the vote, and often say so quite openly.
Oh sure, there are plenty of people that are openly genuinely trying to overturn the vote.
But arguments that the referendum vote for the five words "Leave the European Union" obliges the Lords to reject a policy of prioritising British citizens' rights in the EU post-withdrawal as a negotiation objective could be used as a defining example of laundering policies omitted from the actually voted upon text. Parliamentary bodies imposing conditions on the negotiation process is entirely compatible with the referendum vote and the standard democratic decision making process in the UK, and representatives' votes on them should be determined by whether they think the conditions are reasonable, not by any attempt to infer additional meaning from the five words of the prevailing referendum option.
That's fair. I suspect we're both nutpicking at this point, facilitated by the abundance of nuts on both sides.
I do think it's a little disingenuous to try to limit the mandate to just those four words. If both sides of the referendum campaign made it clear that a Leave win would entail X (e.g. leaving the Customs Union) then it's tough to deny that the result constituted a mandate for X. In the same way, a government that breaks its manifesto promises can't just turn around and say, "Well, we didn't write that promise on the ballot paper itself, so what are you complaining about?"
An abundance of nuttiness around symbolic issues is certainly another argument for why referenda in general make policy laundering easy!
There's a big difference between a manifesto mandate for a government to try to do X, which the Opposition are expected and encouraged to try to revise and thwart, and an assumption that something as little-discussed and likely irrelevant to most people's voting intent as whether to retain a Customs Union is a non-severable, non-contestable facet of public assent to leave the EU. That would have been the case even if a simple, practical customs border solution had been proposed at any time. As it is, we get "look, we can square this circle with the unilateral free trade we've been advocating for years, and to hell with the people who actually voted Leave because they wanted more protectionism not less..."
To be fair, quite a lot of them are.