Aung San Suu Kyi was invited to visit Britain when she met Prime Minister David Cameron in Yangon on Friday.
At the time, she said the fact that she would consider the offer, rather than reject it outright, showed "great progress" had been achieved in Burma.
Suu Kyi, 66, was first detained in 1989, and spent 15 of the next 21 years in detention, refused to leave the country during the brief periods when she was not held by authorities, for fear of not being allowed to return.
Her long refusal to leave Burma characterised her steely determination to defy the ruling junta, which offered to release her from house arrest to be with her late husband, Michael Aris, who died of cancer in Britain in 1999.
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The Office for National Statistics said gross domestic product grew by just 0.1%.
William's comments came during a surprise appearance at an event scheduled for just his brother.
Matt Campbell, who collapsed just a few miles from the finish line, had hoped to raise £2,500.