Nevertheless, Friday's meeting is expected to include talks on the sale of up to 48 Typhoon fighter jets made by British firm BAE Systems - a multi-billion pound deal which has been under discussion for years but has proven tough to seal.
The memorandum of intent was signed after Saudi Crown Prince Mohammed bin Salman met with British defense minister Gavin Williamson.
BAE said the signing was "a positive step towards agreeing a contract for our valued partner".
Downing Street said trade deals worth £65 billion had been agreed during the visit, which were "important both for our economy and the economy of Saudi Arabia".
He added: "If agreed, this shameful deal will be celebrated in the palaces of Riyadh and by the arms companies who will profit from it, but it will mean even greater destruction for the people of Yemen".
"We are committed to supporting the Kingdom as it modernizes the Saudi Armed Forces and develops key industrial capabilities critical to the delivery of Vision 2030".
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But such close ties have displeased some, particularly regarding Saudi Arabia's military intervention in Yemen.
Mr Watkins criticised the welcome given to the crown prince, saying that in Yemen the Saudis were "orchestrating what will potentially become the worst starvation in the last 50 years".
Despite an unusual advertising blitz that saw adverts promoting Prince Mohammed and Saudi Arabia appear in national newspapers and on taxis and electronic billboards across London, the visit was knocked from the British front pages by the poisoning of a Russian double agent.
It said Welby expressed concern about the limits placed on Christian worship in Saudi Arabia, and expressed "distress" at suffering in Yemen, where a Saudi-led coalition is battling Iran-backed rebels.
Later this month Prince Mohammed, 32 will visit the United States, which also wants lucrative ties with the country.
It stressed the importance of reaching a political solution to the crisis in Yemen on the basis of Gulf Cooperation Council Initiative.