"Normally if you cook this with a fried onion and garlic and some spices, it'd be rather nice. As it is, it's... boring. But it's worth it," she told AFP.
Trevelyan is one of 20,000 people in developed nations taking part in a challenge to live for five days below the global poverty line, spending just £1 ($1.55, 1.18 euros) a day on food and drink.
They aim to raise funds for anti-poverty charities and highlight the plight of the one billion people living in extreme poverty around the world.
Trevelyan, a 58-year-old tour guide and actress, was asked to take part by the charity Malaria No More, for whom she hopes to raise £500 in sponsorship.
Before beginning her Live Below the Line challenge on Monday, she planned out her meals carefully, comparing own-brand supermarket prices online to get the best deal.
She has dry toast, an egg and a banana for breakfast and cooks up white rice and frozen vegetables for lunch, sometimes with a discounted frozen vegetarian sausage, but no seasoning.
In the evening Trevelyan eats the rice mix again, or spaghetti, topped with a lurid orange "curry sauce".
"I consider myself pretty broke at the moment... but this was just another whole level. I don't have to buy 26-pence curry sauce," she said.
She also bought teabags but drinks it black, as she couldn't afford the milk.
Her main motivation is to make some money for the charity, acknowledging that her diet this week "is nothing like what a billion people are living on -- this is way better".
She added: "£5 provides a mosquito net big enough to save two children's lives. I just think of that."
The campaign began in 2009 in Australia, where participants try living on AUS$2 a day, before spreading to Britain, the United States (US$1.50), Canada (CAN$1.75) and New Zealand (NZ$2.25).
It has support from Hollywood actors Hugh Jackman and Ben Affleck, and this year hopes to raise up to US$5 million.
But such causes invariably attract accusations of "poverty tourism" by rich people who know they can return to their £2.75 Starbucks latte and organic meat and vegetables after five days.
"We're never going to really know what it's like to live in extreme poverty, we're not kidding ourselves," said Hugh Evans, head of the Global Poverty Project, which organised the challenge.
"But we want to encourage people to have a small taste of what it's like for the bottom billion."
"By the end of the week you realise that food you're eating every single day is monotonous, you're not getting enough basic calories, so you feel that hunger -- you feel it for the first time."
The project is designed to raise awareness of global poverty, but it also shines a light on people struggling closer to home.
"It makes you think about what people here eat -- going around the supermarket, I'm not the only one buying 19-pence spaghetti," said Trevelyan.
Jack Monroe, a 25-year-old single mother from Southend, east of London, hit rock bottom last year after she was forced to leave her job to care for her son Johnny, now three.
She sold her television, clothes and books to pay the rent, turned off the heating and regularly had to feed herself and her son for £1 a day.
Today life is easier -- Monroe's blog about cooking on a budget led to a book deal and a job as a local newspaper reporter -- but she is still taking part in this week's challenge to raise money for Oxfam.
"I'm finding it quite a struggle, to be honest," she told AFP, saying that normally she can use things from her store cupboard, but this week is restricted to a £5 budget.
"I woke up this morning and thought -- I don't want lemon curd on toast again, I don't want cornflakes for my lunch again and I don't want rice for my dinner again," she said.
Monroe acknowledges it is easy to criticise the project.
"A lot of people doing the challenge will be doing it living in their warm, comfy houses and they know they've only got to endure it for five days," she said.
"But I'd never decry anyone who is raising money for charity. And the fact that people are talking about it and thinking about it is the whole point of the challenge -- raising awareness."