The swift rise to fame of Jamael Westman, leading man in the London cast of the hit musical Hamilton, may not equal the extraordinary ascent of the “$10 founding father” he plays in the US show, but it runs close.

Westman who was brought up in Croydon, south London, was not a fan of musical theatre before he landed the part. Now, just three years out of drama school, the 26-year-old, who is of Caribbean and Irish heritage, is seen as a figurehead for black representation in the entertainment industry after 14 months rapping in the starring role. And from this privileged position he is calling for Britain to confront its colonial past. “It is like a mental health issue and it will get worse if Britain doesn’t come to terms with it,” he told the Observer.

Taking over the role of George Washington’s right-hand man – who is depicted on the $10 bill – from the show’s creator, Lin-Manuel Miranda, has had a big impact on the young actor’s politics as well as his career. “I’ve become fairly obsessed about the Hamilton legacy,” Westman said, adding that he believes rap can go some way towards curing the UK’s troubled colonial inheritance. “I have moments of despair about it. But [Alexander] Hamilton was young when he faced these things and he wrote himself out of his situation: rap does the same kind of thing.”

Jamael Westman sits for artist Matt Frost in the Sky Arts series Portrait Artist of the Year. Photograph: Matt Frost/Sky UK

Westman’s response to the growing problem of violence on the streets where he grew up is to criticise cuts to youth services. “I still live in south London and so I feel the connection. For me, it is about engaging with young people. And that’s what Lin-Manuel wants too.”

The star is mourning the death in February of the south London rapper Cadet, real name Blaine Johnson, and said both his music and watching the recent Barry Jenkins film, If Beale Street Could Talk, set in Harlem, made him feel the same way as the lyrics of Miranda’s multi-award-winning show: “It reminds me of a feeling that walks with me every where I go.”

This week Westman is the subject of a series of portrait-painting sessions on the Sky Arts programme Portrait Artist of the Year, presented by Joan Bakewell and Stephen Mangan. Sitting for budding artists had made him think about where he came from, he said.

“I sat there for hours for the portraits, wondering how I got there. Some people are in such a quagmire in south London and I now feel in a position to help alleviate that,” he said. “It is not only the cuts in the funding of youth groups, but Brixton and Streatham have been through a lot of change.”

For the theatrical impresario Cameron Mackintosh, co-producer of Hamilton in London, Westman is what the show needed in the West End. “Because of Lin’s enormous success in the lead role, we knew it would be hard for the cast here if he opened the show just for a few weeks,” he told the Observer. “We needed someone else great and just as original. And we found Jamael in the place we never expected to: Rada. It was not the profile we’d imagined, but he had an interest in rap. It was all completely natural to him.”

Mackintosh has watched Westman change during the run of Hamilton. “I have seen him travel an arch where he has learned you can take things very seriously, and yet still have fun with it. He lights up the stage,” he said.

Westman said he has found “more understanding of the power of the show”. “It’s meant to be cathartic,” he said. “That is what theatre is. But in terms of the ethos of this particular show, we can make it more than a musical. As performers we are the ones who have to do something about explaining togetherness and humanity. And to continue to tell stories that help liberate people.”

•Portrait Artist of the Year is on Sky Arts at 8pm on Tuesday. Hamilton is at the Victoria Palace Theatre until 27 July