Aged seven, Jordan Waller’s biggest obsession was a scrapbook in which he catalogued different types of lesbians. He shows me the book: there are “fat hippy dyke”, “sporty dyke”, “diesel dyke”, “career dyke”, and many more – each sketched in appropriate garb, with their characteristics neatly listed. “Mum absolutely loved it. It was my party piece. We called it the endyke-lopedia, and she’d get me to show it around all her friends.”

Waller, who plays Lord Alfred Paget in the TV series Victoria, struggling with his feelings for another man, grew up with not just one mum but three. “First, there’s my biological mother, Miranda, who got pregnant with me via a sperm donor in 1991,” he says. “Then there’s Dawn, her partner at the time, although they split up when I was about five. And then there’s Jayne, who was Miranda’s partner after that, and who also played a huge role in my upbringing.”

At the close of the TV series, Lord Alfred – having eventually shared a passionate kiss with his amour Edward Drummond – was left bereft when Drummond intervened to save the life of the prime minister, Robert Peel, and was shot dead in the process. But Alfred, says Waller, will “find happiness” in the Christmas special next week.

Growing up with gay parents meant there was real poignancy for him about portraying Lord Alfred. “There was no such thing as ‘gay’ in those times, and it’s been very humbling to think about what that meant for people’s lives, how they had to behave and what they had to work through. What Drummond and Alfred share is ineffable and powerful. The fact that it all has to be secret, unacknowledged, meant I had to approach the role in a very visceral way. I found it all deeply moving, and thought it was very well written. What Daisy [Goodwin, the author] has given us is truthful, as opposed to historically accurate. It’s about what gay people had to go through and how they felt, and you get a real sense of that.”

Bristol in the 1990s was a long way from Victorian London; but decades of gay rights advances had not smashed down all the barriers. “My mothers’ decision to have a child was extremely unusual in those days,” he says. “We didn’t know any other families like ours, and there was a sense of the need to keep ourselves to ourselves.”

Inside their own home, there was lots of laughter. “There was a lot of silliness, of dancing, of dressing up. We had fun. Miranda and Dawn loved having a child, and their friends enjoyed me, too. But the happiness was in a safe bubble. We were leading double lives, we couldn’t be honest about how I’d been conceived or who we were as a family. I’d always been warned not to talk openly about it at school.”

After Miranda (“Mum”) and Dawn (“De-de”) separated, Jayne moved in – and she was a lot more militant, and mouthy.

“One day she came to collect me from afterschool club. The teacher asked who she was, and she said: ‘I’m Miranda’s partner.’ So we were outed, just like that. And then there was a whispering campaign against us: the parents talked about my gay parents, the other kids asked if I was gay. I don’t blame Jayne. In fact, it was a heroic and bold thing to do, but she had no idea how it would all pan out, how people would behave. To me, it felt like being in a nightmare; everything we’d always worried about was coming true.”

Waller moved to a new school a few months later, and the family took a different tack.

“This time, we were upfront and told everyone about our family. I told my friends I had three mums, and that I lived with Miranda and Jayne. And this time the other parents were very welcoming; they had us round to their houses all the time, and I made lots of friends. I guess it was because we were owning it, naming it, and being honest about it. That made a huge difference.”

He kept in close touch with Dawn, spending three days a week at her home. “She was very important to me. She was working class, and very spiritually minded. She gave me a lot of philosophical guidance.”

Sadly, she died last year; Waller describes how he flew home from abroad when he heard how ill she was, and “pelted down the M4” to say goodbye. The months since have been tough, he says: “Nothing can prepare you for it.” One of the few drawbacks to having three mothers is that there are more mothers to lose, and to mourn.

Another difficulty with having three mums is that he felt he was constantly dealing with “a deluge of emotion”. Miranda and Dawn’s breakup left him with a lot to work through. “Like most kids who have separated parents, I got very adept at playing my mothers off against one another,” he remembers. “There was a currency to the emotion, and I was well aware of it.” But his awareness of being ‘different’ proved an advantage. “When you’re different and you know it, there’s often a creative response. You’re forced to explore it and it takes you into new territory and leads you to try out new things.” Like the endyke-lopedia. And also his burgeoning interest in acting. “I was this angelic little thing, with long blond hair, and my mums and their friends feted me, I was the centre of attention, doted on. I was academic and good at drama. I got all the lead parts in the school plays, and my mums and their friends turned out in force to support me.”

When he was 12, Miranda and Jayne split up. Again things were tough. “But as I became a teenager it got easier – I knew myself better, and I was more aware of how to negotiate my position and how to negotiate between the three of them. I felt I was supporting all of them – they were all so important to me. Like Dawn, Jayne kept me in her life – we’re still very close.”

Drama school seemed to beckon, but then someone suggested Oxford. “I was a golden boy and there was lots of excitement at home, from Mum and her friends, about me going off to college. I studied French, but got into drama and went to Edinburgh with shows.” After graduation, he found an agent and got a part in the 2016 Jane Austen movie Love and Friendship as the head footman, Edward. Two series of Victoria followed. He is now writing as well as acting, and has a drama in development about life in a lesbian household, as well as a feature film and a short film in production.

Before then, he will be back in cinemas in January in the first blockbuster of 2018, Darkest Hour, in which he plays Winston Churchill’s son Randolph. Churchill, he thinks, wasn’t a great father. “You get the feeling that the sacrifices he made during the war for Britain were at the expense of his family – there was a lot of fallout.”

He has passionate views about the nature of fatherhood – the role a father provides, the necessity of having a father. He says he doesn’t buy the idea that children need a father figure, or that their lives will be incomplete without one. “It’s a knee-jerk reaction. It bolsters the myth that you need a man and a woman as parents to be whole, and I just don’t think that’s true. What kind of message does it give single parents?”

All he knows about his own biological father is that he is 6ft tall, has dark hair and green eyes. “I was born before the legislation that stopped anonymity for donors, so I’ll probably never know any more about him than that,” he says. “And you know what: that’s absolutely fine. I don’t need to know him. One of the great things about being born to a gay couple is that they have to work hard to have a baby, so you know you were really wanted: babies don’t just ‘happen’ to two women or two men.

“My mothers went to a lot of effort to have me, and I grew up in a very happy home. And no family is perfect. The key thing is to understand that families come in all shapes and sizes, and the important thing is being wanted and being loved, and having your needs met as a child in that family – and mine undoubtedly were.”

As a vote of confidence in the sort of family he grew up in, Waller is now about to become a sperm donor. And, because of legislative changes he is likely, one day, unlike his own biological father, to be in contact with any children born as a result.

“Maybe this is my way to reconnect with ‘Daddy’,” he says. “But all I know is, it felt right. And if I meet my biological children in 18 years that will be great; I’ll be very interested, but the big thing I’ll say is, I’m not going to complete you. I’m not what you ‘need’: you’re getting what you need from the family who raised you, and from what you’re putting into your own life. I’m part of the jigsaw, but I’m not the key.”

The Victoria Christmas special, Comfort and Joy, is on 25 December at 9pm on ITV; Darkest Hour is in cinemas from 12 January