The inauguration ceremony for the 10-foot bronze statue outside the American Embassy in Grosvenor Square honored the deep, personal friendship between Reagan and former British Prime Minister Margaret Thatcher, Sky News reported.
Thatcher, 85, reportedly wanted to attend the ceremony but ill health kept her away, Sky News said.
The inauguration was attended by British Foreign Secretary William Hague and former U.S. Secretary of State Condoleezza Rice.
Rice was representing Reagan's widow Nancy, who at age 90 was too frail to make the trip, Sky News said.
Rice said Nancy Reagan told her: "Ronnie would have been so touched that his centennial birthday is being celebrated in London and central Europe."
Ronald Reagan was born on February 6, 1911.
The statue takes its place alongside statues of other American presidents such as Dwight Eisenhower and Franklin D. Roosevelt.