None of us wanted to go to that reception. It was an industry thing Margaret Thatcher was holding to celebrate London fashion week at 10 Downing Street in March 1984. All the young designers who were invited hated her for everything she had done to the UK. I remember Jasper Conran saying, “Why should I go and share a glass of white wine with that murderess?” I loved him for that. But I realised it was an incredible photo opportunity, whatever I thought of her. So I knocked up that T-shirt a couple of hours before the event. The “58% Don’t Want Pershing” came from a European opinion poll about the proliferation of American cruise and Pershing nuclear missiles across Europe without consulting the electorate, which was totally undemocratic. Wearing that on a T-shirt was the best thing I could think of at the time.

I hid it under my jacket and when they asked to take it at the door, I hung on to it tightly and said, “No, thank you.” We all lined up and the moment I shook Thatcher’s hand, I opened my jacket so the writing would be completely legible to the photographers in the room, who went crazy with their flashbulbs. She looked down and said, “You seem to be wearing a rather strong message on your T-shirt”, then she bent down to read it and let out a squawk, like a chicken.

Katharine Hamnett expressing her view on Brexit. Photograph: Ian Gavan/Getty

Afterwards, I followed her into the cocktail party because I wanted to talk to her about the huge problem of acid rain in Scandinavia as a result of coal burning, but every time I got close she managed to slip away. It was amusing, looking back, like a game of cat and mouse.

The high-fashion protest T-shirts you see today tend to be a bit namby-pamby. They sit on the fence with watered-down messages. I’ve always believed in using media coverage to try to effect change. The reason I was able to do that and say whatever I wanted was because I was always my own boss. We launched my company in 1979 with 500 quid and stayed completely independent. If I wanted to do something, no one could stop me.

My 1989 Clean Up or Die collection – a piece from which will be on display at the V&A’s Fashioned from Nature exhibition on sustainable materials next month – was an example of that. The collection was about sex and subverting Hells Angels messages such as “Too fast to live, too young to die”. I was just finishing it when we got the results of research we had been doing on the impact of the clothing and textile industry socially and environmentally. They revealed what a nightmare it all is, hence the slogan. Little else in the collection was sustainable, but it marked the start of my efforts to clean up the industry by raising awareness of the untold damage done in the manufacturing of materials. It’s super important we address this – it’s great that the V&A’s exhibition is focusing on the sourcing of materials because people need to know.

A million T-shirts later, I’ve come to the conclusion that to really alter things – from climate change to women’s rights – we need legislation. Marches, petitions and fashion statements are all great, but they haven’t worked. They don’t have any teeth. Don’t get me wrong: it’s marvellous that you have people out walking and caring about a cause, but the problem is you’re surrounded by people who think the same as you – which is nice, but it makes you believe the world thinks the same, too, and it doesn’t. All you have done is make yourself feel better and diffuse the energy that could have been put towards making real change.

We need to step it up. I’ve heard that “protest is the new brunch” – great! But we need to follow up protests with pressure: the only thing that changes MPs’ behaviour is if their chances of re-election are threatened. We must tell them they’re paid to represent our views, not take decisions on our behalf: theyworkforyou.com will tell you who your MP is. Ask what they are doing on issues you care about and tell them you won’t vote for them unless they take action. Let’s have that brunch with champagne because we’ve won!

Optimists would say we’re running out of time. Pessimists would say we already have. I’m an optimistic pessimist. I think if we up our game now and use the power of our vote, we can still change the way things are. Political T-shirts are a start. The tragedy is that they are even more relevant now than they were when I started making them, 39 years ago.

T-Shirt: Cult - Culture - Subversion is on at The Fashion and Textile Museum until 6 May 2018.