A lifejacket that has been used by a refugee to cross the sea to Europe is laid out in Parliament Square Getty Images

If you grew up in the British colonies – as I did – you tend to be a little sceptical of jingoistic grandstanding, just as African-Americans are when their country declares itself the supreme, most godliest of nations. British values? American dreams? Christian morality? What do those mean to people whose lands were stolen or whose ancestors were taken from their homelands?

In the UK and US today, there is much triumphalism, much big talk about greatness and uniqueness. Donald Trump and Theresa May project that national narcissism, albeit in very different ways. Both extol the defining virtues of their lands. Deceptive egotism and convenient myths obscure unpalatable truths.

Think about slavery, war crimes and exploitation. Uganda, my birthland, is a perfect case study. In 1970, Britain, the US and Israel backed a military coup that got the monstrous Idi Amin into power. (Read Unpeople; Britain's Secret Human Rights Abuses by Mark Curtis, 2004).

Two years later, Amin expelled Ugandan Asians from their country. Many had British passports. Millions of xenophobic Britons, instigated by Enoch Powell, wanted to keep us out. Ted Heath, after some hesitation, admitted us. That ended his political career. When I interviewed him, he said he had no regrets about doing the right thing. He didn't understand that doing the wrong thing brings easy popularity to politicians. It was ever thus.

Trump wants to build a wall to keep out Hispanic migrants, to stop Muslims from travelling to the US, to revoke the civil rights legislation. His son, Donald Trump Jr, just posted a tweet comparing Syrian refugees to a bowl of fruit-flavoured sweets: 'If I told you just three could kill you, would you take a handful? ' Clever lad. Takes after his colourful, toxic dad.

May also wants to keep out distraught, broken refugees from Syria and other troubled places. Children dying in the sea, or languishing in Calais, don't seem to move her, nor do women weeping with their faces behind barbed wire fences. They are simply numbers, not names, not humans, not children of god, which she, as a practising Christian, should at least recognise.

On Saturday, thousands of Britons, who haven't yet misplaced their humanity, marched through London. They included people of all races and backgrounds: Vanessa Redgrave, BBC presenter Anita Rani, Lord Alf Dubs – who has campaigned indefatigably for child refugees.

Daniel Harris, 35, from London walked despite being on crutches; a seventy-three-year-old from Southend on Sea turned up. She was outraged that our government makes wars easily but does not take any responsibility for the victims. A tearful old Jewish man who came here as a child said he was doing his duty. On Monday, to coincide with the UN meeting on the global refugee crisis, protesters laid out 2,500 life jackets worn by refugees in from of Big Ben.

And what was Mrs May's response? She strode into the UN, lectured them on how it was best to keep victims in or near places where they are being bombed, tortured, starved and shot. "We have always taken the view that we can help more Syrian refugees by putting aid into the areas," said she, without a blush.

In Syria, where Assad stops aid getting through to the dying? And aid from Priti Patel, the international development secretary, who previously proposed abolishing this department? Hardline May and her right-wing friends want to bring in workers we need, but do nothing for people who need to work and make a future, just as Europeans have done over centuries.

Remember, 86% of refugees live in the developing world. Turkey, Jordan, Lebanon, Tanzania, Kenya and Pakistan support millions of refugees. The deeply honourable Angela Merkel has taken a million refugees. Now the mayors of New York, London and Paris have together called for empathy and relief for asylum seekers and economic migrants: "Our cities stand united in the call for inclusivity. It is part of who we are as citizens of diverse and thriving cities."

We signed up to refugee and human rights conventions. We are now breaking those pledges. Duplicity doesn't get more duplicitous than this.

At least Trump doesn't try to come across as principled. May does and then behaves abominably. When Holocaust memorial day comes along, I am sure we will hear her speechifying about the UK's proud history of sheltering refugees, the great success stories of those who settled here and other warm, meaningless stuff.

But yet when it comes to today's migrants and refugees desperately seeking life chances, she will not hear the pleas of caring citizens or asylum seekers. She has chosen to be on the side of fiercely anti-refugee and anti-migrant Britons – who include settled refugees and migrants. Appalling. Incidentally, Priti Patel is the daughter of Ugandan Asian exiles too.

David Cameron had some compassion. Our new PM has none. If Mrs Thatcher was the Irony Lady, Mrs May is steel. If her views and plans prevail, they will bring shame on our nation. We signed up to refugee and human rights conventions. We are now breaking those pledges. Duplicity doesn't get more duplicitous than this.