“KBO is the motto for the day, I’m afraid,” Nicholas Soames says, sighing. The long-serving Conservative MP and grandson of Winston Churchill is quoting his grandfather’s favourite acronym. It stands for “keep buggering on”.

Soames, who is 69, has brought the phrase into the 21st century using his Twitter account. The hashtag “#KBO” is just one of his many idiosyncratic pronouncements that have contributed to his social media fame.

In a classic of the genre aimed at Donald Trump, Soames wrote, “Such boring infantile whingeing and whining, just grow up, ignore it and do the job or go,” in an unpunctuated, one-word hashtag. Eurosceptic Tories are his main targets: he called the MEP Daniel Hannan a “wetty” and instructed the MP John Redwood to “#buggeroff”.

“I think it is a wonderful toy,” he tells me. “I’m quite sure that, had Twitter been of an age when my grandfather was prime minister, I’m sure they would have used it… He was a brilliant and gifted communicator.”

The British Bulldog glowers at us from all angles of Soames’s Westminster office: a black-and-white portrait on the wall, a bronze bust on the cluttered shelves, a painted oval box on the writing desk where we sit.

Today, Soames, a former army officer, wears his grandfather’s grimace. Leaning back in his chair, his hands plunged deep into his pockets, he is feeling melancholic. A golden candle burns with the scent of “Spiritus Sancti”, giving the room a hushed atmosphere. Soames, an ardent Europhile, served as a defence minister under John Major. He has experienced Tory highs and lows since he was elected in 1983 as the MP for Crawley (he now represents Mid Sussex), but he has never known a period like this.

“It was a catastrophe,” he says of the June general election. “It was a woeful campaign. By far the worst of the nine that I’ve fought, by a country mile.”

He reveals that he warned people “around the Prime Minister” that they should lay off Jeremy Corbyn. “These constant ad hominem attacks were a disaster and weren’t working, unless you were 40 years old or above… People looked at us and thought, ‘The Tory party are not good enough for us.’”

He sips from a slim, pink can of rhubarb fizz – the man once unkindly nicknamed “Fatty Soames” has lost a lot of weight over the past year – and paces the room. His navy braces are patterned with a skull-and-crossbones design, and his black hair is severely slicked back: an Old Etonian in the guise of an East End gangster.

“If the Prime Minister was good enough to ask me what I thought about anything,” he booms, “I would say to her, ‘Do not be defined by Brexit. Don’t allow this to dominate the whole of the government.’”

He wishes that his party would embrace “One Nation Toryism” – meaning unionism, openness and a “sound” economy. “We can’t be all things to all men,” he warns. “At the first whiff of grapeshot, some members of our party seem to have run away. I don’t think, frankly, Boris or Michael Gove are in order at all to start shouting the odds about public-sector pay. They need to learn and understand restraint. It’s not their place to say it.”

He is most exercised by the Brexit debacle, which he says will have “near-fatal results” for Britain. Following the EU referendum, Soames was so horrified that he changed his mind about standing down and continues to represent his constituents in the Commons.

“We look like we’re not the country we were,” he says. “I worry greatly. We don’t have a skilled economy. We are way behind on our technical training. This is a poorly educated country, frankly.”

Soames has recently returned from Calais, where new statues of Churchill and Charles de Gaulle were unveiled. An admirer of the new French president, Emmanuel Macron, he believes that France is recovering its self-confidence, whereas we are “losing ours”.

“I’m not optimistic for my country,” he adds. “I don’t see how we’re ever going to project our influence and our standing and our power outside the EU… Britain’s voice is going to diminish, I’m afraid.

“We’re going to have to rebuild our entire diplomatic effort… We’re going to need to refortify the Foreign Office, which has been hollowed out over many years… We can’t go on running our defences down. We can’t have a smaller army than we’ve got now. It would be fatal. The navy is already a husk of its former self.” He pauses. “I’m sorry to sound like Colonel Sir Bufton Tufton.”

Soames has been accused of being an out-of-touch toff. He was pictured riding a horse on the campaign trail. “What’s wrong with that?” he splutters. “There’s so much chippiness. I’m sorry if people don’t like it. I enjoyed it.”

Are there too many Old Etonians in politics? “I simply couldn’t give a monkey’s about where people went to school,” he says. “I think this country is genuinely a meritocracy.”

Soames was recently chastised by the Speaker for barking at the former SNP MP Tasmina Ahmed-Sheikh.

“I regret having done it, OK?” he says now. “It wasn’t a sort of ‘coo-err’ woof. She deliberately made a point of it. It didn’t do any bloody good. She lost her seat. Her voice got higher and higher, like a little fox terrier. I only went, ‘Woof!’ – like that…” The candle flickers at his breath. “I’m not going to pretend to be anything I’m not.”

What Nicholas Soames is not is a bore. In fact, he is one of the great Commons characters. Rising from his desk, Soames walks me out of his office. “Don’t murder me,” he says.