The London office, in the suburb of Cricklewood, is being run by relations of two of Morsi's arrested aides, who were seized with the former President last July.
One of the relations, who asked to remain anonymous, said they viewed London as a "safe city" and "the capital of a free democracy that values human rights and social justice".
Another said: "We look forward to seeing those values brought back into Egypt once our democracy is restored and our freedom from dictatorship and repression won."
On Thursday, Egyptians will be asked to take part in a referendum to endorse a new constitution that will entrench the military's powers and enable it to continue to curb the Brotherhood. Campaigning against the constitution has effectively been forbidden and it is almost certain to be approved.
Ibrahim Mounir, a leading figure in the group's international wing, who is also based at its London office, issued a warning to the regime that it would not last. "All military coups must come to an end," he said.
In Cairo, the Brotherhood's ability to function is in tatters, with more than 1000 supporters killed during confrontations with Egypt's security services since July.
The Government accuses the Brotherhood of turning to violence after Morsi's fall, and formally designated it a terrorist organisation on Christmas Day. The Brotherhood says it is committed to peaceful protest and there is scant evidence linking it with militant groups.
Mounir admitted operations in Egypt had been severely hampered, but said it was little different from the Brotherhood's life under previous strongmen, such as Hosni Mubarak and Gamal Abdel Nasser.
"We have been used to this [repression] for 60 years, and wecan still function, albeit in a different way to before," he said.
Referring to calls from Brotherhood supporters for the leader of the military coup, General Abdelfattah al-Sisi, to face the death penalty, he said: "It is our objective that the coup leaders are held accountable for their atrocities."
Key constitutional changes
Egypt's more than 52 million voters will go to the polls to decide whether to approve the country's rewritten constitution, which limits the scope of Islamic law and introduces new articles seen as a victory for rights advocates. It also expands the powers of the military in politics. Here are some of the key changes.
Civilian government
The draft states that the charter "continues to build a democratic, modern country with a civilian government". The phrase "civilian rule" had stirred anger among ultraconservative Islamists who consider it synonymous with "secularist". Some liberal constituent panel members accuse the head of the panel of changing "rule" to "government" to appease Islamists without telling them.
Islamic law
The new charter retains Article 2, which says the "principles" of Islamic law are the basis for legislation. However, it removes a Morsi-era provision that gave a more precise definition for "principles" that could have been used to enact stricter Islamic law. It also deletes a reference to a role for Al-Azhar, the country's main Islamic institution, in overseeing legislation.
The military
A key clause gives the armed forces the right to name the defence minister over the next two presidential terms, an arrangement that places the military above any civilian oversight for eight years. Rights advocates say the new charter fails to ensure any level of transparency for the armed forces' budget or details of its economic interests. Civilians can still be tried before military tribunals.
The President
The draft gives the president the right to appoint a prime minister and gives parliament two chances to support the president's choices, or be disbanded. For the first time, parliament has the power to remove an elected president and prosecute him over a list of crimes.
Political parties
The draft prohibits political activity or the establishment of political parties based on religion.
- AP
