The African leader has recently faced questions at home over whether he is well enough to run the country.
Since the start of 2017 he has spent more time in the UK than in Nigeria.
His current period of sick leave is the second this year, after a stint from January to March. The nature of his illness has not been disclosed.
"I feel I could go home, but the doctors are in charge," Mr Buhari said in a statement. "I've now learnt to obey orders, rather than be obeyed."
He added that there had been a "tremendous improvement" in his health.
Some protesters in Nigeria's capital, Abuja, have demanded the president's resignation this week. Others called for more transparency about his condition.
When Mr Buhari first went to London for treatment in June 2016, his office said it was for a persistent ear infection.
His main opponents in Nigeria's 2015 election had claimed he had prostate cancer - which he denied.
Photos released by the presidency show the 74-year-old standing outdoors in sunglasses and smiling as he holds a "Get well soon" card.
Mr Buhari has appointed his deputy, Yemi Osinbajo, to carry out his duties in his absence.
The leader's medical challenges have sparked speculation in Nigeria about whether he will be fit to seek a second term in 2019.
If he does not stand for re-election, observers say, it could prompt tensions between factions in the largely Muslim north and mostly Christian south.
The health of Nigeria's leaders has been a sensitive topic since 2010, when President Umaru Musa Yar'Adua died in office after months of treatment overseas.
Nigeria's President Muhammadu Buhari has said he feels ready to return home after medical treatment in the UK, and is awaiting his doctor's permission.