Britain has signed a multi-billion-pound preliminary order from Saudi Arabia for 48 Eurofighter Typhoon fighter jets, military equipment maker BAE Systems said on Friday.
Britain's Prime Minister Theresa May greets the Crown Prince of Saudi Arabia Mohammad bin Salman outside 10 Downing Street in London.
"This is a positive step towards agreeing a contract for our valued partner".
At the start of the trip, the countries set themselves a 65 billion pound ($90 billion) trade and investment target for the coming years, with London looking for new post-Brexit markets for its services sector, and Prince Mohammed seeking to convince wary investors his country is a tolerant and modernising place.
That has provided a controversial backdrop to the warm diplomatic welcome for Prince Mohammed, with hundreds turning out on Wednesday to protest arms sales and Saudi Arabia's role in Yemen's civil war. It pits Saudi Arabia's Sunni Muslim monarchy against Yemen's Houthi rebel movement, which is backed by Shiite Muslim power Iran.
The UK government has faced criticism from opposition leaders and campaigners about selling military hardware to Saudi Arabia during the ongoing humanitarian crisis in 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".
Save the Children's chief executive Kevin Watkins warned Saudi Arabia is targeting children in the brutal war in Yemen with a "growing sense of impunity".
Speaking in London at the launch of The War On Children, a report into crimes against children in warzones, Mr Watkins said: "It has become acceptable to operate humanitarian blockades which, if not explicitly created to starve children and harm children, will have that inevitable outcome".
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.
The 32-year-old crown prince has shaken up his deeply conservative country, pushing through dramatic social reforms, including allowing women to drive, and waging a massive crackdown that the ruling family says went after corruption, but critics say simply targeted his political foes.
"We're all concerned about the appalling humanitarian situation in Yemen", May said, adding she meant to raise "concerns about human rights when I meet him".