William Hague, the first British foreign minister to visit Myanmar in over half a century, met with Suu Kyi for about one-and-a-half hours following a dinner with her and meetings with senior government members on Thursday.
After the talks in Yangon, he praised promising steps that have been made since the nominally civilian government took power in March but said "much more needs to be done" if restrictive European Union measures are to be lifted.
"It is not possible to say a country is free and democratic while people are still in prison on the grounds of their political beliefs," he said.
Hague also called for free and fair by-elections in April and improved humanitarian access to ethnic conflict areas, saying it was "very important that we do not relax the pressures prematurely".
The EU and the United States both impose economic sanctions on Myanmar over its human rights record, including the imprisonment of hundreds of political prisoners.
Speaking on the last day of his trip, Hague described himself as a "great admirer" of the Nobel peace laureate Suu Kyi and her struggles for freedom and democracy.
"This is a very exciting time because there is a chance that what she and her colleagues have hoped and longed for for so long will actually take place in this country."
Myanmar has played host to a series of top international envoys in recent months after controversial 2010 elections heralded the end of decades of direct military rule.
Suu Kyi, who has grown cautiously positive about Myanmar's future recently, said she expected to live to see a "full democratic election" in Myanmar, in comments to the BBC before her dinner meeting with Hague.
After meeting with him again on Friday, she said she looked forward to the time when Britain and Myanmar's relationship improves and becomes "mutually beneficial".
The Nobel laureate is on course to run in April 1 by-elections after her National League for Democracy (NLD) was officially allowed to register as a political party on Thursday.
Suu Kyi, who was freed from seven straight years of house arrest days after a November 2010 election, could be propelled into parliament by the upcoming poll, although the majority held by the army and ruling military-backed party will not be threatened.
Since taking office last year Thein Sein -- himself a former junta general -- has surprised observers by holding talks with Suu Kyi, suspending an unpopular Chinese-backed dam project and indicating a desire to reach out to the international community.
Some political prisoners have also been released but the government this week caused disappointment when it announced reduced jail terms for inmates but failed to issue a much-anticipated amnesty for detained dissidents.
Hague's trip, the first by a European Union foreign minister since the new government took power, is the latest round of international diplomacy aimed at urging on the budding reforms.
"I made clear that the British government stands ready to respond positively to evidence of further progress towards that lasting improvement in human rights and political freedom that the people of Burma seek," he said in a UK Missions statement following talks with the president.
Hague's visit echoes that of US Secretary of State Hillary Clinton, who met Thein Sein and Suu Kyi during a trip to Myamar at the end of last year.