Bukola Saraki on Wednesday travelled to the British capital with the majority leader in the Senate, Ahmed Lawan, and the speaker of the lower House of Representatives, Yakub Dogara.
It was the latest political delegation to visit 74-year-old Buhari at Nigeria's official diplomatic residence, where he has been staying for nearly a month.
Saraki said in a statement on his Facebook page that Buhari was "healthy" and there was "no cause for alarm". The head of state was "cheerful and in good spirits", he added.
In a separate series of Tweets, he added there was "no vacuum in government" and "all organs of government (are) fulfilling their mandate".
A tweet on Buhari's own Twitter account late on Wednesday said he appreciated the visit and thanked "Nigerians, Christians and Muslims alike, for their prayers and kind wishes for my health".
Unlike some heads of state and government, Buhari does not initial tweets he writes himself. Photographs showed him meeting the politicians looking apparently relaxed.
Buhari's health has been the subject of increasing concern in Nigeria since he extended his stay in London earlier this month, just as he was scheduled to return to Abuja.
The presidency has repeatedly insisted he is not seriously unwell but was awaiting the results of medical tests.
The former army general, who headed a military regime in the 1980s, received treatment for what his office said was a persistent inner ear infection in London in June last year.
The health of Nigeria's president is a sensitive issue given the death in 2010 of president Musa Umaru Yar'Adua from a long-standing, but previously undisclosed, kidney complaint.
His initial illness and treatment in hospital abroad triggered months of political uncertainty. His deputy, Goodluck Jonathan, took over on Yar'Adua's death.
Buhari's number two, Vice President Yemi Osinbajo, is deputising during his absence.