Sam Gyimah, the softly-spoken former Tory minister, admitted he was still adjusting. It was just a few hours before the unassuming MP for East Surrey was unveiled as the Liberal Democrats’ latest recruit at the party’s Bournemouth conference. But it all felt very new.

“I think I’ll probably spend the next few days saying, ‘Sam Gyimah, Liberal Democrat MP’, and get used to that first,” he said.

“There is a huge emotional side to it. I’ve been involved in the Conservative party for two decades. I’ve fought for the party. I have an unusual background – I’m not your typical Tory recruit. I’ve spent a long time evangelising about why people should look at the Conservative party seriously. It is sad that I find myself at a crossroads. I am an outcast in the Conservative party. But that’s Brexit. It has divided families. The country is divided. This is a huge fault line.”

It wasn’t long ago that Gyimah was a Tory rising star, flying up the ranks during Theresa May’s tenure and fast-tracked on to the party’s frontbench.

Yet here he is, still just 43, starting afresh with Jo Swinson’s party. The Tory party he joined, he says, is rapidly disappearing.

“Born in the UK, brought up in Ghana, it was a sort of childhood of hardship and difficulty,” he said. “What attracted me to the Conservative party was these One Nation values. What has happened since the referendum, initially slowly, but increasingly accelerated over the last few months, is that it has moved away from those values. It has become more intolerant. It has become, in the desire to neutralise Nigel Farage and the Brexit Party, more doctrinaire.

The Conservative party has become more intolerant and, in the desire to neutralise Nigel Farage, more doctrinaire

“I listen to ministers undermining the courts. Ministers questioning experts because their views are inconvenient for what the government is saying about no deal. Or you have a government that says law enforcement is the centrepiece of its platform, and yet says in another breath that it will pick and choose what laws it chooses to respect. This is in many ways undermining key pillars of our constitution and the functioning of our democracy.

“The issue for me is not just Brexit. It is beyond Brexit – how you conduct politics and the veering towards populism and English nationalism.”

Gyimah made headlines at the end of last year when he concluded that he could not support Theresa May’s Brexit deal, meaning he had to resign as universities and science minister. He said more serious concerns about the party emerged during the Tory leadership campaign, when he ran in order to try to create a debate about the party’s direction.

“I thought it was an opportunity, while the party was discussing its future, to make the case for Brexit realism, and also the case for rediscovering these values. During that process I realised there just weren’t enough people who were going to fight from within to save the situation. As we saw soon after, not only did [Boris] Johnson win an overall majority on a no deal platform, but we very quickly went from a situation where the leader of the party had to be a no dealer, to anyone in government having to sign up to it. That was playing on my mind throughout the summer.”

His precarious place in the Tory ranks was brought to an end, along with that of 20 of his colleagues, when he voted to block a no deal Brexit earlier this month. That was his signal to try something different.

“The withdrawal of the whip meant I just had to face a stark reality – being in the Conservative party now means you’re signing up to no deal or hard Brexit, or you’ll be cast into the wilderness. So what are your choices? Leave politics completely?

“It feels to me that there is a scorched earth approach to delivering Brexit by 31 October at the risk of our institutions, at the risk of our democracy, at the risk of our economy. That should not be allowed to happen.”

It feels to me that there is a scorched earth approach to delivering Brexit by 31 October

Gyimah says all 21 rebels received letters from the chief whip offering them a way back in – but the letters differed. He explains: “There were three different versions of the letter. I think the one I got was the lower tier.

“Whatever the letter said, I think I knew that just being realistic about the situation, I couldn’t take the whip back because I will not be able to satisfy the minimum price of entry.

“But I don’t want to spend all my time dwelling on that. The other side is, what do you do about it? That is why I’m here. Centrists are being cast out of both main parties. Lots of people are politically homeless. Who can you work with to build a movement?

“When I spoke to Jo I thought this is a project that I can be part of. I’m not looking for a harbour in a storm. I’m attracted to a project to defend liberal democracy and make sure our country does not drift into the nationalistic, populist politics that we see in other democracies.”

Does he have a message for the 20 other Tory MPs thrown out for trying to block no deal?

“My message is not just to the 20 – my message is to all of those who care about this country. The time is now. The problem we have is that the two old parties have entrenched votes and it is very difficult to budge them.

“We need the centre to consolidate and unite if it is going to break through. This is the opportunity to try to create that new politics for the good of our country.

“There’s no reason why the settlement that comes out of Brexit should be the settlement that Nigel Farage wants.

Dominic Grieve and Sam Gyimah at a ‘People’s Vote’ rally in April 9. Photograph: Jack Taylor/Getty Images

“It would be incredibly complacent for us to assume that our institutions and our politics are immune from these [populist] forces. We can bemoan the Labour party and the Conservative party. That is all well and good. But we have to act and that is why I am here.”