This is Britain: Three years, 120 towns, 800 portraits - how the photographer Niall McDiarmid captured the state of the nation

Like any number of good ideas, this one arrived during a stroll. On a winter Sunday in 2011, Niall McDiarmid took the afternoon off from his family and went for a walk. The photographer ventured to the South Bank; he ended up chatting to people along the way and asking if he could take their photo.

So he started heading off elsewhere, to places such as Guildford and St Albans; any town that "you wouldn't normally associate with a photographic project".

Influenced by Daniel Meadows, who carried out a similar project in the 1970s, and Joel Sternfeld, McDiarmid took hundreds of train journeys, visited 120 towns and photographed 800 subjects over three years to produce Crossing Paths, his portrait journey around the UK. It manages to be both a personal project – the photographer struck up conversations with anyone he thought interesting, enabling him to capture something of their personality in the portraits – as well as a document of modern Britain.

For while dozens of styles and ages are featured, such a collection of characters could come from nowhere other than here, reckons McDiarmid. "I was interested in people who had a certain Britishness about them – though exactly what that is, is hard to pin down. I'm also interested in how similar our high streets tend to be – you get the same shops in every town – but the people who walk round them seem to be very different.

"I have this feeling that fashion – and by fashion I mean what people wear – is much more broken up than it used to be. In the 1970s, 1980s and early 1990s, people tended to wear a particular style of clothing, whether it was flares or big collars. Now, people wear all sorts of different things, and they feel comfortable wearing them." k

Bright colours explode from the pages of his book, from backgrounds, from clothing and – with a plethora of tattoos on show – from his sitters' skin, too. "We tend to think of this country as having fairly dull weather. I wonder whether people try to compensate by wearing brighter clothing? It's the same with the recession. Do people tend to wear more colours during hard times?" It was one of the reasons that McDiarmid decided to use an old medium-format film camera ; he found the colours showed up better than on digital. The camera also, he adds, usefully acted as a good conversational starting point.

McDiarmid found that a number of towns were not quite what he expected. "I had preconceptions about places, as many people do. Hull, for example: a lot of people would say that it's a bit of a dull town without ever having gone there. But I found really interesting people there and they were very friendly. Norwich, meanwhile, was booming, and there were a lot of creative people there."

Spending anything from 15 minutes to a few hours in each town, McDiarmid approached locals by asking them questions about the area and their lives. "I'd find out their story then tell them about the project and ask if they fancied it. Some people said no but most would be up for it."

Since the project also has a website, many of the pictures go up online, and the internet has allowed the images to spread. McDiarmid often receives messages from people telling him they recognise someone's portrait. Perhaps they are an old teacher, or a family friend. More often than not they were a character in the town that people remember fondly. "I'm very interested in this idea of people having a home town," he says. "People might move away but they'll always have ties to that place. You can't get away from your home town."

'Crossing Paths: A Portrait of Britain' by Niall McDiarmid is available from crossing-paths.co.uk, priced £22.50 plus P&P