James Frayne is Director of communications agency Public First and author of Meet the People, a guide to moving public opinion. The focus of this column is Theresa May’s conservatism for “ordinary working people”.

Post-Brexit, we’re going to have to reset the nature of the foreign policy conversation that has come to dominate in Britain. This is one where our politicians are encouraged to think in absolute terms about a country’s ethical record and their Government’s outlook – and to form a British response accordingly.

What began as a welcome shift in Britain’s approach to diplomacy and foreign policy – with an increasing acceptance that ethics matter – has started to detach some politicians and NGOs from the reality of securing the national interest. While Labour were right to talk about the need for an ethical foreign policy in the late 1990s, too many people now see diplomacy as an extension of domestic culture wars.

We’ve seen this in the last week as Theresa May prepares to fly out to see President Trump. Some are already effectively calling for the Prime Minister to pick a fight with him over his domestic political agenda. While I am sceptical about the merits of forging close personal links with someone like Trump, it would be completely mad to criticise him publicly for policies that he was voted in to enact and won’t change. Britain needs to do a deal with Trump; that clearly won’t happen if Theresa May launches some sort of attack on him.

We will shortly hear the same sorts of things as Theresa May prepares to visit China. She will be encouraged to raise concerns about China’s record on human rights and democracy. And, again, it would be ridiculous for May to do so. The Chinese won’t change their behaviour in the slightest, whatever Britain says, and Britain needs the prosperity that will result from closer trading links. George Osborne was right about our links with China.

This is not to advocate for the return of’ Perfidious Albion’ and a values-free foreign policy. Not only would that be the wrong thing to do in principle, it would also undermine Britain’s ability to use soft power in the world. Countries are more influential if their values and motives are respected.

But we need to be realistic about our current situation – and realistic too about the ability of Britain to change the world. Now that we’re leaving the EU, we not only need to secure deals with major powers like the US and China but with a whole mass of countries who we may or may not agree with politically or approve of morally. In the past, the EU took care of all such matters for us; now we face the harsh reality of conducting deals with countries across the world ourselves. Without those deals, Brexit is going to be an uncomfortable experience.

And while it is true to say that Britain still carries weight in international politics – partly because of our history, partly because of our professional network of diplomats and other advocates – we have to accept our limits. Every country has its own domestic audiences and its own geopolitical priorities. Sometimes they can be influenced, sometimes they can’t. There’s no point trying to fight what can’t be changed.

Theresa May should go out to Washington and get some business done. That’s not about endorsing Trump, that’s about endorsing Brexit.