John McTernan is head of political practice at PSB, a strategic research consultancy. He was a speechwriter to ex-British Prime Minister Tony Blair and was communications director to former Australian Prime Minister Julia Gillard. The opinions expressed in this commentary are his own.

(CNN) It is normally an iron law of politics that there is always a deal to be made.

This rule holds particularly true when it comes to negotiations between nations.

All countries employ talented, intelligent and inventive diplomats to ensure that there will be an agreement.

So why is this not the case when it comes to Brexit? It is not that there is a lack of political understanding of the possibility -- indeed the need -- for a deal, at least on the part of the 27 countries that will remain member states of the European Union after the UK leaves.

Nor is there a lack of talent on the UK's side. The negotiating team is brilliant, committed and energetic. Or at least the civil servants are: The problem is the politicians.

I remember taking my son -- then four -- to Prime Minister's Questions in the House of Commons in London. He observed in silence the parliamentary give and take from the Visitor's Gallery. But afterward, when asked by his Member of Parliament, Tessa Jowell, what he had thought of the whole thing, he responded thoughtfully: "If we behaved like that at our nursery we would have time out."

The baying, the catcalling, the noise was all to him bad behavior that cried out for discipline. For a long time politics restricted its childish behavior to these limited set pieces. But that is no longer true. The only way to understand modern British politicians as they address Brexit is to realize that they have infantilized politics.

Firstly, there is the banality of the discourse. Former senior civil servant Sir Martin Donnelly quipped that leaving the European Union because trade might be better outside was "giving up a three-course meal [now]... for the promise of a packet of crisps in the future."

Faced with this well-expressed and logical point, International Trade Secretary Liam Fox responded that Brexit is "a little more complex than a packet of Walkers."

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In another era, the refusal to engage intellectually and instead to resort to something more worthy of the playground would have been roundly mocked. But standards have slipped so far in British politics that these days there appears to be stiff competition for the most laughable statement of the day.

Earlier that same day, Foreign Secretary Boris Johnson had compared the border between Ireland and Northern Ireland -- over which violent war was fought -- to the one between the London boroughs of Camden and Westminster, which not even local councillors could accurately sketch. It is all a depressing reminder that the main quality currently required for senior political office in the UK is narcissism.

Secondly, that narcissism -- writ large -- is the driving force of the positions of political parties.

The Northern Irish Democratic Unionist Party -- which props up Theresa May's minority Conservative government -- is using its position both to extort money from the Prime Minister and to champion a so-called hard Brexit -- ignoring Northern Ireland's vote to stay in the EU.

Meanwhile, the Scottish Nationalist Party is using the issue to argue for a second independence referendum.

The Conservative Party uses it to have a collective nervous breakdown -- slowly and in public -- while its fringe of europhobic Brexiteer backbenchers pushes for a bigger, bolder, madder, faster Brexit.

And what about the opposition Labour Party? You would have to be a saint not to want to exploit the writhing agonies of the government and the leader of the opposition Jeremy Corbyn never misses an opportunity to kick the government when it is down.

Some of this is entertaining, but most of it is horrifying. Britain, after all, faces its single most important decision in peacetime. Do the political parties show any recognition of this? Do they show that they care about the country, its people and its businesses? Of course they don't. This is a narcissist's paradise. It's all, and always, about them.

The tragedy is not simply in the outcome -- a bad Brexit means a Britain that is permanently poorer. But it is also a tragedy in the process -- because there is a practical and pragmatic deal to be done.

Could Britain live being out of the EU, but in the Customs Union and the Single Market? It would mean Britain would have to take rules from the EU and contribute to its budget.

But we would be out of the EU as the referendum demanded. A compromise? Yes, but in a very British way, we would be making the best of things and muddling through.

Will it happen? It should. But how can we get there? Maybe my son was right all those years ago: Childish behavior needs the sanctions of a good parent. British politicians -- of all parties -- need to be made to sit on the naughty step until they decide that they should work together to get the best deal for everyone in Britain, rather than the deal that positions them closest to the front door of Number 10 Downing Street.