The former trade minister Lord Price is unswayed. He voted remain and is still a remainer, despite spending a year at the heart of the government Brexit machine. “I voted to remain and if I voted again today I’d vote to remain,” he explained. With the experience of more than a year in the trade department, criss-crossing the world banging the drum for Brexit Britain, he reckons the UK has up to five years of “bumps along the road” ahead.

“The UK had a remarkable set of opt-outs and opportunities,” Price told the Guardian as he prepared to take an unpaid role as chairman of the Fairtrade Foundation.

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“We were able to leverage not being in the euro, not being in the Schengen Area and had opted out of some of the social chapters,” he said. “Over many years the UK had engineered a position where it was a great place to come and do business. We have now got to think how we can achieve the same but in a global context.”

Price had spent the previous decade running the supermarket chain Waitrose and was hired by the then prime minister David Cameron in 2016. But within months he found himself working for the arch Brexiteer Liam Fox, who was installed by Theresa May as international trade secretary.

Price continued: “I am optimistic that we can get to a good place in terms of an independent trading relationship, but it will take three to five years. It won’t happen overnight and in the interim companies might think twice about investing and consumers might decide they want to be more cautious.”

Given that roughly 30% of what is eaten in Britain today is imported from mainland Europe, there are concerns about the outlook for grocery prices if British negotiators cannot replicate the status quo.

Price is optimistic, however. “I don’t think European countries want to impose tariffs on food. At the moment there is a £10bn surplus in food traded with the UK. France runs a surplus of £4bn, of which £3bn is food. Spain equally [runs a surplus], Italy equally. Those countries are very aware of the fact the UK is extraordinarily important to them.

I don’t think European countries want to impose tariffs on food

“We are adopting EU standards on day one, so if there is regulatory compliance and no tariffs – and goods can be checked at the point of departure and arrival – I wouldn’t expect to see food rotting at the ports. With a transition period, there is enough time to plan.”

Price joins Fairtrade at a time when the charity is said to be facing the biggest crisis in its 25-year history. Its famous logo is stamped on more than 5,000 products, ranging from coffee and tea to bananas and flowers, but its supporters were stunned last year when Sainsbury’s, the world’s largest retailer of Fairtrade products, announced it was setting up its own foundation.

The supermarket giant’s decision follows that of the US group Mondelēz International, the owner of Cadbury, which has developed its own certification scheme called Cocoa Life. Some supporters worry that the proliferation of company-run ethical schemes will confuse consumers and weaken a Fairtrade model that guarantees a minimum price to farmers as well as a financial bonus used to fund community projects.

Price argues that the opposite is true: “Fairtrade doesn’t survive or not because of the decision Sainsbury’s make about putting their own mark on a box of tea. It’s bigger than that. It’s a huge idea about helping the least developed [countries] get a fairer deal.

“How can Fairtrade be in crisis when it’s got a turnover of £1.6bn in the UK and a business like Mondelēz is saying it wants to work hand in glove with it and Sainsbury’s is saying it’s so important they might put in place the investment to build their own mark? Ultimately the goal of Fairtrade is not to have Fairtrade.”

These days the Fairtrade brand is part of the commercial mainstream and, despite the Sainsbury’s setback, sales increased by 7% to exceed £1.7bn for the first time in 2017. However, Price cautions: “If you think about Fairtrade as just a commercial entity and that’s it, I don’t think that’s right. It also has to be a voice for change and fairness.”

As the charity prepares for its 25th birthday, the Tory peer, who has also turned his hand to writing business and children’s books, said it was time to consider how the Fairtrade model was evolving.

“Do people pay us to help them certify products, or for the infrastructure we have?” were, he said, among the questions he would work with the charity’s chief executive, Michael Gidney, to answer. Price believes another area to explore is how Fairtrade products could be sold online.

The businessman, who spent more than 30 years working for the John Lewis Partnership, also plans to use his experience of the corridors of power to ensure that farmers in developing countries are not penalised by Brexit.

“There is a role to play in making sure our voice is heard in the department for international trade (DIT),” he says. “I also want to make sure we have a strong relationship with the department for international development.”

The EU has carved out economic partnership agreements with a number of African, Caribbean and Pacific countries, and Price said it was important for the UK to “pick those up” and offer countries the same deal they have now. “There are some developing African countries paying tariffs on agricultural goods coming into Europe, and the DIT has said they would want to take some of those away. It’s about making sure we are close to that.”