Saudi Crown Prince Mohammed bin Salman, Deputy Prime Minister and Minister of Defense, was being warmly greeted by Queen Elizabeth II, as he began a three-day visit to Britain on Wednesday.
May insisted she would raise human rights issues when she sat down with Bin Salman, but added: "The link that we have with Saudi Arabia is historic, it is an important one and it has saved the lives of potentially hundreds of people in this country".
Al-Shorouk quoted Prince Mohammed as saying "the contemporary triangle of evil comprises Iran, Turkey and extremist religious groups".
Both countries 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.
May also said the United Kingdom is "holding the Saudis to account", adding: "We have encouraged the Saudi Arabian government to ensure that when there are allegations of activity taking place which is not in line with worldwide and humanitarian laws they investigate it and learn lessons from it. That's why, as a Government, we have increased our funding for Yemen".
Saudi Arabia has waged a long military campaign in Yemen in support of the internationally recognized government that Houthi rebels drove out of the capital, Sanaa.
As the 32-year-old prince's auto drew up to the Prime Minister's home, protesters thronged the vehicle - with one man arrested for throwing an egg during the protest. London also eyes listing state oil firm Saudi Aramco in its stock market.
Opposition Labour lawmaker and shadow Foreign Secretary Emily Thornberry criticized the red-carpet welcome given to the Crown Prince in an opinion piece for Britain's Guardian newspaper in which she highlighted the civilian toll of Yemen's civil war.
The prince spoke to Egyptian newspaper editors during a visit to Cairo, on his first foreign trip since becoming heir to the oil exporting giant previous year.
The 32-year-old prince is a contentious figure, in part due to Saudi Arabia's role in the current war in Yemen, with air strikes carried out by a Saudi-led multinational coalition killing thousands of Yemeni civilians.
"Behind the smiles and handshakes on show tomorrow, and despite Prince Mohammed bin Salman's attempts to be a poster boy for progress, there are some uncomfortable truths about his repressive government that must not be forgotten", she wrote in an editorial for The Independent.
Business deals and initial agreements could be concluded on gas exploration, petrochemicals and other industries, according to British and Saudi sources.
Questioning the desert kingdom's treatment of dissidents, human rights defenders - as well as accusing the Government of "colluding in what the United Nations says is evidence of war crimes" - Mr Corbyn said: "As she makes her arms sales pitch, will she also call on the crown prince to halt the shocking abuse of human rights in Saudi Arabia?"
The Muslim Association of Britain warned against entering into trade with Saudi Arabia "at the expense of essential values" and said recent "superficial" reforms by the Saudi government should not be taken at face value.
Demonstrators are protesting against Britain for licensing 4.6 billion pounds of weapons sales to Saudi Arabia since 2015.
The UK government has come under intense pressure over its sales of arms to Saudi Arabia amid concern they could be used in Yemen in breach of global humanitarian law.