The red carpet has been rolled out at Buckingham Palace and No. 10 Downing Street in London for Saudi Arabia's Crown Prince Mohammed bin Salman.
The jets, made by British company BAE Systems, are part of a multibillion-pound deal which has been under discussion for many years. Britain credits Saudi intelligence sharing with saving British lives and has licensed billions of pounds of weapons and ammunition sales to Saudi Arabia.
"Commenting on Friday's provisional deal announcement, the group said that this was a "positive step towards agreeing a contract for our valued partner".
This included talks with Prime Minister Theresa May, the Archbishop of Canterbury, and an audience with the Queen.
The Saudi delegation will meet May and senior ministers inside May's Downing Street offices to launch a UK-Saudi "Strategic Partnership Council" - an initiative to encourage Saudi Arabia's economic reforms and foster cooperation on issues such as education and culture, as well as defence and security.More news: Daylight saving time 2018: Things to know
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". It pits Saudi Arabia's Sunni Muslim monarchy against Yemen's Houthi rebel movement, which is backed by Shiite Muslim power Iran.
Before his visit, the 32-year-old Crown Prince Mohammed bin Salman had emphasised the importance of the relationship between the United Kingdom and Saudi Arabia regarding security.
Ahead of his visit, the crown prince signalled the importance of the UK-Saudi Arabia relationship to security, saying both countries would be "much safer if you have a strong relationship with Saudi Arabia".
Protests have followed the crown prince, also called MBS, as he travels the UK.
May intends to use the private dinner at Chequers on Thursday, a 16th-century manor house 60km northwest of London, to bring up concerns over the humanitarian crisis in Yemen, her spokesman said.