Campaigners against the war also rallied near parliament and held a protest outside the gates of Downing Street.
Britain is rolling out the red carpet for Crown Prince Mohammed bin Salman, who will have lunch with the queen at Buckingham Palace and later dine with Prince Charles and Prince William.
Britain's planned exit from the European Union did not reduce its attractiveness as an investment destination, the minister told reporters, and he said he expected the crown prince's visit to take bilateral relations to a higher level. He also helped institute sweeping changes for women, including allowing them the right to drive for the first time in the kingdom's history.
May responded: "The link 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". That's not really the actions of a terribly reforming leader, is it?
According to Downing Street, the meeting will tackle worldwide challenges, such as terrorism, extremism, the conflict and humanitarian crisis in Yemen and other regional issues such as Iraq and Syria.
"The British and Saudi people, along with the rest of the world, will be much safer if you have a strong relationship with Saudi Arabia", he said.
Mohammed bin Salman, known as MBS, posed with the Prime Minister outside 10 Downing Street on Wednesday afternoon ahead of talks between the pair. Theresa May is handing a major propaganda coup to the dictatorship.
Announced in October by the energetic de facto ruler, Crown Prince Mohammed bin Salman, Neom will try and attract foreign investment to cultivate new industries including water and energy, food, media, advanced manufacturing, biotechnology and entertainment.
The demonstration was organised by Avaaz, a civic platform that among other causes promotes human rights.
Does the Saudi coalition have worldwide support?
"The UK has been totally complicit in these abuses".
Speaking after he questioned the prime minister, Stewart McDonald MP said: "The Prime Minister has an opportunity today to raise the Saudi regime's appalling human rights failures. No matter how bad the conflict has got, May and her colleagues have continued pouring weapons into it".
Leaders from both the nations will launch "UK-Saudi Strategic Partnership Council" - an initiative to encourage Saudi Arabia's economic reforms and foster more cooperation on issues such as education and culture, as well as defense and security.
"This could be the real transformation, not just for Saudi Arabia but for the whole region".
Al-Jubeir provided no further details on the agreements nor on the precise agenda for the crown prince's visit.
"Saudi Arabia is on the verge of a crucial new stage of modernity thanks to the efforts of Saudi Crown Prince Mohamed bin Salman", former Assistant Foreign Minister Ambassador Gamal Bayoumi said.
"In a Saudi context, these changes have vast significance".
Andrew Smith, of Campaign Against Arms Trade, said: "Theresa May might believe the Crown Prince's ridiculous claims that he is a reformist and a force for liberalism, but people in the United Kingdom are not so easily convinced".