President Cyril Ramaphosa on Tuesday received two letters from Queen Elizabeth that the late former president Nelson Mandela had written her.

The exchange took place during his visit to Windsor Castle.

"She was reminiscing about her relationship with Nelson Mandela, particularly on the year when we celebrate 100 years of his birth," he said after the visit.

"She remembered very fondly the discussions she used to have with him. She also gave me two letters that he wrote to her, wonderful pieces of memorabilia."

The letters marked South Africa's re-entry to the Commonwealth in 1994, according to the Royal Family's official Twitter account.

Ramaphosa is in the UK on a working visit to London to attend the Commonwealth Heads of Government Meeting, which will take place on Thursday and Friday.

He met with British Prime Minister Theresa May for a bilateral meeting and agreed on a deal worth £50-million (R857-million) to help South Africa improve its business environment.

"We focused largely on the economy and how our partnership between SA and the UK can be deepened around economic matters, the economic growth we are trying to drive," Ramaphosa said.

He said May was "quite enthusiastic" about assisting and wanting to find ways in which to play a role.

Ramaphosa said they would be trawling "the whole world" to try and campaign for investments in South Africa.