Britain's grand welcome for Saudi Crown Prince Mohammed bin Salman begins on Wednesday with a lunch with Queen Elizabeth, as the two countries seek to widen longstanding defence ties into a far-reaching partnership.
Both sense an opportunity to broaden their existing relationship: Britain is looking for trading partners as it exits the European Union, and Saudi Arabia needs to convince sceptical investors about its domestic reforms.
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But as Prince Mohammed and Prime Minister Theresa May meet, demonstrators will protest against both countries' roles in Yemen where war has killed an estimated 10,000 people and where 8.3 million people depend on food aid and 400,000 children have life-threatening levels of malnutrition.
Foreign minister Boris Johnson led the welcoming party for Prince Mohammed on his arrival late on Tuesday. Wednesday's first official engagement is a trip to Buckingham Palace for talks and a meal with the British monarch - a rare honour usually reserved for heads of state.
After lunch, the Saudi delegation will meet May and her cabinet inside May's Downing Street offices to launch a UK-Saudi "Strategic Partnership Council" - an initiative to encourage Saudi Arabia's economic reforms and foster cooperation on issues such as education and culture, as well as defence and security.
"It will usher in a new era of bilateral relations, focused on a partnership that delivers wide-ranging benefits for both of us," May's spokesman told reporters.
Britain is vying to land the stock market listing of state oil firm Saudi Aramco, but no decision is expected this week.
Later this month Prince Mohammed visits the United States, which also wants the lucrative listing, although sources said both countries may miss out.
British officials were privately delighted at the decision by Prince Mohammed, 32, to choose Britain as the major western destination on his first foreign trip since becoming heir to the Saudi throne last year.
The British government is keen to transform its historic defence relationship into two-way trade and investment, eyeing both an expanded market in Saudi Arabia for service sector exports, and attracting Saudi cash to finance domestic projects.
Business deals are possible with British defence group BAE Systems and European weapons maker MBDA, and initial agreements could be concluded on gas exploration, petrochemicals and other industries, according to British and Saudi sources.
"This modernisation will not be easy, nor will it be something we can do alone," Saudi Arabia's ambassador to Britain, Mohammed bin Nawaf, wrote in the Financial Times.
"We will need to tap the expertise of others. So, as we transition away from our historic reliance on oil, enormous commercial avenues will open up for overseas companies to work with, and invest in, Saudi Arabia."
The three-day visit will include two audiences with the British Royal family, a briefing with national security officials, and a prestigious visit to the prime minister's country residence.
May intends to use the private dinner at Chequers on Thursday, a 16th-century manor house 40 miles (60 km) northwest of London, to bring up concerns over the humanitarian crisis in Yemen, her spokesman said.
A Saudi-led military coalition is fighting the Houthi movement in Yemen, generating what the United Nation said in January was the world's worst humanitarian crisis.
"You can expect them to discuss Yemen, and the prime minister to raise deep concerns at the humanitarian situation," May's spokesman said. "She will also reiterate how seriously we take allegations of violations against international humanitarian law."
Speaking to reporters in London on Monday, Saudi foreign minister Adel al-Jubeir said his country had failed to effectively communicate the reasons behind its involvement in Yemen, but that they had not chosen to start the war.
Protesters are planning to target the Saudi officials over Yemen and other human rights issues, and Britain for licensing 4.6 billion pounds of weapons sales to Saudi Arabia since 2015.
"As so often, it is about nothing but filthy lucre, and this government's desperation to plug the hole that will be left in Britain's trade and growth prospects by May's refusal to stay in a customs union with the EU after Brexit," Emily Thornberry, foreign policy chief for Britain's opposition Labour Party, wrote in the Guardian newspaper.
Buses have spent two days touring London with banners accusing Prince Mohammed of war crimes, with more planned for Wednesday before the main rally.
"Theresa May might believe the Crown Prince's ridiculous claims that he is a reformist and a force for liberalism, but people in the UK are not so easily convinced. This visit is being done to legitimise a brutal dictatorship and to sell arms," said Andrew Smith of the Campaign Against Arms Trade.