Courts insist that witnesses tell the truth, the whole truth and nothing but the truth. Partial truths can often conceal a small fib, while whopping big lies can often come wrapped around a small kernel of truth.

Politicians should be made to take a similar oath. The UK prime minister, Theresa May, for example, recently insisted Britain "must get the best possible trade deal with the EU from its upcoming Brexit negotiations". That sounds eminently sensible and is perfectly true. But it (deliberately?) conceals the deeper truth that Britain already has the best possible trading arrangements with the European Union: it's called the single market.

Anything that changes this best possible deal will, by definition, be worse.

Philip Hammond, the UK chancellor of the exchequer, recently claimed that the UK electorate "didn't vote to be poorer on 23rd June". That's not even close to the truth. Fifty-two per cent of voters most certainly did vote that way: the biggest single impact, so far, of Brexit has been a 15 per cent fall in sterling, something that unambiguously makes every UK citizen poorer.

Psychologists talk about the five stages of grief. Victorious Brexit campaigners often sneer at Remain voters, telling them to get over their denial, anger and depression. A whole new branch of psychiatry is going to have to be invented for people who win a wholly unexpected victory who then become utterly delusional about what it means.

Crocodiles

Brexit is really about the internal battles of the ruling Conservative Party, one that goes back decades. Ken Clarke, a former Tory chancellor, recently described May's dilemma in terms of that struggle: as she paddles her canoe along the treacherous Brexit river, trying to keep everything stable, crocodiles in the form of the swivel-eyed loon wing of the party are circling her. She can keep throwing them treats to keep them at bay, but one day she is going to run out of supplies. The appetite of the crocodiles is insatiable; and once she runs out of treats they will move in for the kill.

The first prime minister to run out of treats was David Cameron: his last gift to the lunatic wing was, of course, the referendum. May's announcement of a Brexit date will keep the crocodiles away for a while. But they are circling, waiting for the day that she admits to a compromise of one kind or another with the EU. Seen in this light, it is a bunch of ideologues in complete control of British policy: the needs of the economy don't matter. It's a funny way to run a country.

The news that article 50 of the treaty on European union will be triggered by the end of March has been greeted with a further fall in sterling. Every time one of the Brexit ministers makes a speech displaying no sense of awareness of economic reality (which is every time they open their mouths) sterling falls.

Brexiteers are displaying delusion in multiple ways. They talk about just quitting the EU and reverting to World Trade Organisation (WTO) rules. The fact that this would be a highly complex exercise has obviously escaped them: Britain is part of the WTO via its membership of the EU and it is not clear how or when it could be admitted into the WTO. They don't seem to understand that nobody has negotiated a trade deal with the EU in less than seven years. The EU now has every reason to drag out the negotiations and leave them utterly unfinished when Britain has to leave the EU, some time during the first quarter of 2019 – that's the hardest Brexit imaginable.

Favourite delusion

Brexiteers console themselves with their favourite delusion: the EU needs the UK so much that Brussels will inevitably make big concessions. This does not even contain a kernel of truth: UK-EU trade is much more important to the UK; and, in any event, politics, not economics, will drive EU policy.

The focus on immigration is becoming more intense and more illiberal. May seems to want the NHS to employ only British people. The more you think about that sentence, the weirder it becomes. We are told that restrictions on skilled non-EU immigrants are on the way, something that will harm British productivity, growth and employment. Similarly, according to the education secretary, we can look forward to fewer students being allowed into Britain, harming one of the few high-growth, high-productivity sectors.

The single biggest delusion is the idea that it is all going to be simple. Already we hear of negotiations over Britain’s share of the EU wine collection stored in Brussels cellars. The list of things to haggle over may take over two years to compile, let alone discuss.

Meanwhile, the French central bank has set up a Brexit hotline (in English, apparently) to assist any UK-based financial institution thinking of upping sticks. Other European institutions, regions and cities (including Dublin) are also actively targeting London for businesses thinking of relocating. Just like the foreign exchanges, they notice the utter incoherence of the Brexit ministers.

Brexit negotiations currently amount to a discussion held by the Tory party with itself. When the real talking starts, the British side will have revealed all its cards – a spectacularly weak hand.