For those like me who never heard the term:

Estuary English is an English accent, continuum of accents, or continuum of accent features[4] associated with the area along the River Thames and its estuary, including London, since the late 20th century. In 2000, the phonetician John C. Wells proposed a definition of Estuary English as “Standard English spoken with the accent of the southeast of England”.[5] He views Estuary English as an emerging standard accent of England, while also acknowledging that it is a social construct rather than a technically well-defined linguistic phenomenon.[5] He describes it as “intermediate” between the 20th-century higher-class non-regional standard accent, Received Pronunciation (RP), and the 20th-century lower-class local London accent, Cockney. There is much debate among linguists as to where Cockney and RP end and where Estuary English begins, or whether Estuary English is even a single cohesive accent.[5][6][7][8]