When people talk about the decline of the British Empire, rarely do they mention the love story that played a significant part.

The marriage of Seretse Khama, heir to the throne of Bechuanaland, to white British typist Ruth Williams in 1947 paved the way for independence for modern day Botswana.

It is a remarkable true story that is brought to life in south London director Amma Asante’s film A United Kingdom, which is out November 25 and opens the 60th BFI London Film Festival tonight.

The political fall-out from the interracial nuptials was enormous but for Asante it was important that this was explored through the story of the romance.

Speaking at the London Film Festival’s opening night press conference, Asante said: “For me the love story allows you to bring the intimacy to the story.

“The backdrop for me should always be political in some way or big world in some way. That is what both (Asante’s previous film) Belle and A United Kingdom bring in many ways. It was very important to me that the balance was right.”

She added: “Nothing in this story should come without it passing through the prism of this couple’s love.

“So, for instance when you have (Prime Minister) Atlee talking in the corridors at Parliament and he explains his predicament he talks about all of his reasons for why he cannot let that couple set foot on Africa and sleep in the same bed together.

“Even when they are not in the same scene it is very important that it comes through the prism of this couple’s love. That is what I tried to hold on to throughout.”

A United Kingdom was shot partly on location in Botswana and in Asante and star David Oyelowo features a black female director and a black lead so clearly it is a triumph for diversity in an industry which still has a long way to go.

Oyelowo, who was the driving force in bringing the project to the screen, said: “All we are seeing here with A United Kingdom is a reflection of the country we live in.

“Amma directing this film shouldn’t be special. It’s special to me artistically but it shouldn’t be special that a woman – if women represent 50 per cent, by and large, of the population – [is directing this film]. But here we are, we are still in that place.

“My hope is that when people see this film they will see themselves in both Ruth and Seretse. They will see this country’s history.

“They will see us as people of African descent’s history and how that intersects with British history inextricably and why we are very proud to call ourselves British and why we are very proud to call ourselves Africans and with time it will become less and less special.”

His co-star Rosamund Pike, who plays Ruth Williams, said it was important to challenge the assumption by distributors that A United Kingdom should be categorised with films such as 12 Years a Slave and Mandela.

She said: “Really the goal would be for this to be seen in the canon of love stories and the protagonists and the colour of their skin should be irrelevant in terms of genre and how a film like this will fare in the cinema.

“It should be given parity and compared to films that are more like it in subject matter.”

Asante added: “Oftentimes when we go to the cinema the majority of the films that are on at the cinema happen to be about men, happen to be about white men and they happen to be within a particular age bracket.

“When we talk about diversity on screen obviously that is what we are challenging.

“We are not saying we want to stop those films being made of get rid of them, we are simply saying that there are other realities that are also default experience.

“I walk a female path every day, I see the world through these female eyes and that’s my default experience.

“There are 50 per cent of other human beings in this country that walk a similar path to that.

“It is not about removing what is already there, it is about allowing the space for others to join and have the same privilege.”

A United Kingdom is out November 25. The BFI London Film Festival runs until October 16. Go to bfi.org.uk/lff

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