Every vote counts, but some count a lot more than others. When British voters go to the polls on Thursday, the great majority of them will do so knowing the outcome in their constituency is a foregone conclusion. Out of 650 Westminster constituencies, according to the Electoral Reform Society, a pressure group, 365 (56 per cent) are so "safe" that nobody but the incumbent has any real chance of winning.

The first-past-the-post system has long served up such certainties: in 12 of the 17 general elections since 1950, fewer than one in 10 seats shifted from one party to another. Some have remained firmly in one party’s control for more than a century.

With the shape of the next government likely to come down to the results in the minority of constituencies – marginal seats – with a history of changing hands, it is in these places that the parties are targeting their efforts and their money. More than 40 million people may be eligible to vote, but the ballots that really matter are those of about 135,000 swing voters in the most marginal areas.

There are few places where the outcome looks more assured than in Richmond, a vast constituency that covers large parts of rural north Yorkshire. This is a relatively wealthy electoral area, made up mostly of quiet villages and hamlets across a swathe of countryside that takes in much of the Yorkshire Dales national park, Wensleydale and part of the North Yorkshire Moors to the east.

Safest Tory seat

If Richmond’s scenery has changed little in the past century, its politics have changed even less. The Conservatives have held the constituency continuously since 1910 and, based on figures from the last election, when former party leader

William Hague

won by more than 23,000 votes, it is the safest Tory seat in Britain.

"Of course I'll vote," says Brian Kent, a retiree and lifelong Conservative voter in the town of Richmond, where tea shops and outdoors suppliers look onto a cobbled market square. "We know the outcome, but if people don't go out and vote then that won't be the outcome, will it? Better the devil you know than the devil you don't know."

Hague's decision not to contest the seat he has held since 1989 has made Richmond a little less predictable than usual. The man the Conservatives have selected to succeed him is Rishi Sunak, a 34-year-old HQ-approved investment fund entrepreneur who has been parachuted into the safe seat.

"He's clearly not a local," says Andrew Wilson, who is out shopping with his wife. "I think there might be a bit of a backlash, and I suspect Ukip might gain some advantage."

It's widely assumed that Sunak will struggle to retain anything close to Hague's vote, but a defeat seems unthinkable. "If the Tories lose here, they're in dire trouble," says John Golds, who moved to Yorkshire from London 30 years ago and has always voted Labour.

The campaigns have a perfunctory air in Richmond. There are few of the roadside billboards and hoardings that cover more competitive constituencies, and many voters say they haven’t even seen a candidate on the campaign trail. On Sunday, four days before polling day, no party canvassed the busy market square.

“I would imagine the Conservatives don’t need to [campaign] and Labour feel they’d be wasting their time,” says Kent. This tallies with national data on British parties’ election spending, which shows that the cash value of an individual’s vote varies wildly depending on where they live.

Analysing spending at the 2010 election, the Electoral Reform Society, which lobbies for reform of the first-past- the-post system, found that the money spent on winning a single vote varied between £3.07 in competitive races and 14p in ultra-safe seats. Some 348 candidates spent no money at all on their campaigns, including four Conservative candidates, four Labour and 20 Liberal Democrats. “2010 was a tale of two elections – and two electorates”, the society concluded in a report. “One that mattered, and one that didn’t. And for the majority of us the election was over on day one of the campaign.”

A shoo-in

Despite their constituency’s predictability, however, Richmond residents still come out to vote in large numbers. Turnout here in 2010, at 67.2 per cent, was higher than the national average of 65.1 per cent. Voting is “something that you always do,” says Golds. He accepts the Conservatives are ”probably a shoo-in”, but he plans to vote for the Labour candidate, Mike Hill, even though he doesn’t think he has a chance (in 2010, Labour got 8,000 votes in Richmond, the Tories got four times as many).

So why vote at all? The paradox is that, although many people can be fairly certain of the outcome in their constituencies, the law only holds if they actually go out and cast their supposedly pointless vote. Even the safest seats can spring surprises. In the past 35 years, five of the 20 safest seats in the UK have changed hands. George Galloway took a Labour stronghold in 2005, as did Jim Murphy in the Scottish Eastwood constituency in 1997. In the same year, the Conservative Michael Portillo gave his name to the spectacle of a safe seat being swept away by a huge swing: the "Portillo moment".

Richmond-born Andrew Wilson, who voted for Tony Blair’s New Labour and then the Liberal Democrats but has yet to decide who to support this week, insists it’s vital to cast a vote even if it can’t swing a result.

"Everywhere in the country, the numbers are counted. So if I voted for the Greens, for example, they wouldn't win the seat, but that vote goes towards their national figures and shows where they stand."

The popular vote for each party may be even more significant than usual in this election, given that a hung parliament would make questions of legitimacy and mandate key factors in the coalition-building process. But above all, as Golds sees it, casting his vote is about being “responsible” and taking part in a wider national drama. “Election night is like a cup final, isn’t it? I want to shout at the screen angrily like everyone else.”