Crown Prince Mohammed bin Salman's first day in London saw him lunch with Queen Elizabeth, visit the Churchill War Rooms and meet with Theresa May at Downing Street, as a fierce political row over United Kingdom military support for Riyadh threatened to overshadow the Saudi royal's first official visit to the UK.
The new investment includes an education agreement, a Downing Street spokesman said, as protesters shouted outside amid tight security.
She argues "it is about nothing but filthy lucre" and that rolling out the red carpet is shameless.
UK Foreign Secretary Boris Johnson also greeted the Crown Prince at the bottom of his plane when he landed on Tuesday night.
Although May noted that she would raise humanitarian concerns about Yemen in his meeting with bin Salman, she stressed that all arms sales to Saudi Arabia were strictly regulated. Later Wednesday Prince Mohammed was due to be hosted for dinner by heir-to-the-throne Prince Charles, with Prince William among the guests.
Britain is looking for trading partners as it exits the European Union, and energy powerhouse Saudi Arabia needs to convince skeptical investors about its domestic reforms.
Asked whether Bin Salman should have been invited to the U.K., May said the two countries had a historic relationship and one which had helped to potentially save hundreds of British lives - presumably because of anti-terrorism intelligence shared between the countries.
Labour leader Jeremy Corbyn accused the government of "colluding" in war crimes by supplying arms to Saudi Arabia. "This vindicates the engagement that we have with Saudi Arabia, to be able to sit down with them", she said.
It follows a trip to Egypt earlier in the week, when Prince Mohammed met Coptic Pope Tawadros II in Cairo.
In the past, Turkey has also worked with Iran, which is Saudi Arabia's arch-enemy in the Middle East, to try to reduce fighting in northern Syria.
Since January 2015, Yemen has been ravaged by a bloody civil war with Saudi Arabia backing the country's Sunni elites and Iran supporting the Shia Houthi rebels.
So could a Saudi woman ever become the chief executive officer of Aramco - the giant state oil company that's planning one of the largest initial public offerings in history?
May's Conservative government said Mohammed's visit would strengthen co-operation in "tackling worldwide challenges such as terrorism, extremism, the conflict and humanitarian crisis in Yemen and other regional issues".
According to the Campaign Against Arms Trade (CAAT), over the past three years, Britain has licensed $6.4 billion worth of arms sales to Saudi Arabia. As the British government officials have pointed out, this has in turn supported "tens of thousands" of United Kingdom jobs.