The victim was Sophie Lionnet, a 21-year-old French au-pair from Troyes in north-eastern France. She had been employed to look after the couple's two children.
Officers were called on Wednesday evening to a house in Wandsworth, south-west London "following reports of a fire and a person deceased", a statement said.
"A body was found at the scene. The body was too badly burned to establish the gender and age. A post-mortem examination will be held in due course."
A 40-year-old man, Ouissem Medouni, and a 34-year-old woman, Sabrina Kouider, were charged on Friday morning for murder of an unknown person.
The pair lived on Pulborough Road, which leads off Wimbledon Park Road, where the body was found at a residential address.
Neighbours had become alarmed at the "foul-smelling smoke" coming from Kouider's garden and a young boy reportedly raised the alarm after he scaled the apartment's back yard gate and saw a huge bonfire with a man throwing what he thought were 'sticks' on the flames.
A friend from her school in Paron, to the south of Paris, told British tabloid The Sun that Sophie was a "happy, socially aware young woman who everybody loved".
According to neighbours, Ms Lionnet, who had been living in London for 14 months in order to learn English, had not been seen for weeks.
There are suggestions she was deeply unhappy and had been due to fly back home to France on Monday but never arrived.
Thousands of young French women, many school leavers, work as au pairs in London, where there is a 300,000 strong French community.