British Prime Minister David Cameron listens to a final speech by former Foreign Secretary William Hague on day one of the Conservative Party conference in Birmingham, central England, on Sunday. Agence France-Presse/Getty Images

BIRMINGHAM, England—U.K. Prime Minister David Cameron will seek to rally his Conservative Party as it meets this week for its last annual autumn conference before next year's general election in the midst of lawmaker defections and public discontent over the economic recovery among the challenges he faces.

Adding to the pressures on the prime minister, he has committed Britain's military to joining international airstrikes against Islamic State in Iraq, a decision Parliament approved on Friday but one that has already raised concerns about whether the mission will spread to Syria and when it will end.

U.K. Royal Air Force Tornado combat jets began flying armed missions over Iraq on Saturday.

Although the U.K. economy is growing at a solid pace again, many Britons have yet to feel better off after several years in which consumer prices have risen faster than wages.

The main opposition Labour Party has accused the government of fueling a "cost-of-living crisis" by focusing too much on austerity policies to cut the nation's budget deficit.

Mr. Cameron will look to convince party members that the government's economic plan will pay dividends at the ballot box, and that the Conservatives have the right policies on immigration and the European Union to stave off the electoral challenge from its small anti-Europe rival, the U.K. Independence Party.

With the Conservatives divided over Britain's membership of the European Union, UKIP is threatening to split the right-wing vote in 2015 and make it harder for Mr. Cameron to win a second term with its strident opposition to Britain's EU membership and tough immigration stance.

Mr. Cameron speaks during a television interview with Andrew Marr at the BBC in Birmingham on Sunday, ahead of the Conservative Party's annual conference. Reuters

Over the past month, two euroskeptic Conservative lawmakers have defected to the small party—the most recent one, Mark Reckless, announcing his decision on Saturday. Furthermore, political analysts expect the first of the two to jump ship, Douglas Carswell, to win back his seat on the east coast of England in a by-election on Oct. 9—Mr. Cameron's birthday. That victory would give UKIP its first elected member of the U.K. Parliament and an invaluable national platform from which to attack the Conservatives.

Mr. Carswell defected to UKIP in August, saying the Conservative leadership wasn't serious about changing Britain's relationship with Europe.

Mr. Cameron and his Treasury chief, George Osborne, will hope to unite the party by focusing on the U.K.'s recovering economy.

"It's the economy that builds houses. It's the economy that creates jobs. It's the economy that pays for hospitals. It's the economy that puts food on the table. That's why it's the economy that settles elections," Mr. Osborne is expected to say in his conference address on Monday, according to extracts of the prepared speech.

In an effort to appeal to older voters and savers generally, Mr. Osborne plans to announce that the government will offer a tax break on the pension pot of a deceased person before it passes on to their beneficiaries, according to extracts of the speech. The measure takes effect in April and will cost about £150 million ($244 million) a year.

With eight months until a general election, the Conservatives trail the Labour Party in opinion polls. A survey of voting intentions of 1,992 adults released by polling firm YouGov PLC on Sunday showed Labour, led by Ed Miliband, on 36% and the Conservatives on 31%.

"The broad issues are going to be things like immigration, what happens with Europe, and the economy, which I think the Conservative Party has a very good story to tell. But clearly the impact of the achievements of George Osborne and the Conservative Party's economic strategy is not being felt by people at grass-roots level and is certainly not being reflected in the polls," said Ben Harris-Quinney, chairman of the Bow Group, a Conservative think tank.

Adding to the negative news for the Conservatives on the eve of the conference, Brooks Newmark, the party's minister for civil society, announced on Saturday that he was quitting his cabinet post over a report that he sent explicit pictures of himself to an undercover reporter posing as a female party activist. Mr. Newmark couldn't immediately be reached for comment on Sunday. On Saturday, he released a statement through the party appealing for the privacy of his family to be respected.

"I have to admit it hasn't been the ideal start [to conference]," Mr. Cameron told the British Broadcasting Corp. on Sunday. "But the truth is these things, frustrating as they are, they don't change the fundamental choice in this election, which is do you want to continue with the long-term economic plan that is working and that can deliver for Britain's families and hardworking taxpayers, or do you want to lurch off with Ed Miliband, with no plan, no leadership, no ideas about the economy?"

Another issue for Mr. Cameron stems from the recent Scottish referendum on independence, after which he offered to transfer more powers to England to match those he offered to Scotland during the campaign if they opted to stay part of the United Kingdom—which they did. While that promise has been welcome by many in his party who want only English lawmakers to have a say in English matters, putting the pledge into action could prove complicated.

Despite the Scottish victory, Mr. Cameron got into trouble in the days after he was recorded telling former New York Mayor Michael Bloomberg that Queen Elizabeth II "purred" with happiness after he told her that Scotland had voted to stay in the U.K., breaking an unwritten convention that discussions between the prime minister and the monarch are private. The Queen was officially neutral on Scotland's independence referendum.

"It was not a conversation I should've had, even though it was a private conversation, and I'm extremely sorry and very embarrassed about it," Mr. Cameron told the BBC.

Write to Nicholas Winning at nick.winning@wsj.com and Jenny Gross at jenny.gross@wsj.com