The term you're looking for is "Standard British English" or "Standard English." This is the neutral and non-regional form of British English that serves as a reference point for pronunciation, grammar, and vocabulary. It is sometimes also referred to as "Received Pronunciation" (RP), although RP specifically focuses on the pronunciation aspect. Standard British English is often used in education, the media, and formal communication and is not associated with any particular region or accent. Regarding native Londoners, it's essential to note that London is a diverse and multicultural city with a wide range of accents and dialects. Many Londoners may use Standard British English in formal or professional settings, but they can also have accents influenced by their specific neighborhoods or backgrounds. Londoners can have Cockney, Estuary English, Multicultural London English (MLE), and various other accents or dialects. There is no single "London accent" that applies to all native Londoners. The prevalence of Standard British English may vary depending on the context and the individual's background.

There’s a term known as “estury English” I coule be wrong but I believed this is the most common form of British English

I agree with what Ahmadi has said, although I would add, for the avoidance of confusion - Received Pronunciation is associated strongly, though not exclusively, with the south-east of England, especially London and the counties surrounding it. So it is a major accent in London, one of the many. But its strongest association is with class: with the middle and upper classes, who, wherever they are in the country, are much more likely to speak with RP than working-class people. That is especially true (again) in London and the south-east, but even as far away as Scotland, the nobility often speak just as people do in the south-east of England. Because of that I am extremely sceptical about your 3% figure. I would be very surprised if it's much less than 15%. As for the most common accents, I would say there are seven major accent groups in the UK: 1. RP, as described above 2. "Estuary" English, which is (very roughly) working-class people in the south-east of England 3. "Northern", which includes dozens of varieties in Yorkshire, Lancashire etc. etc. 4. Westcountry, which is most strongly associated with the south-west of England but can actually still be found as far east as Sussex 5. Welsh, which again is divided into lots of different accents 6. Scottish, ditto (look up Doric on youtube for a really impenetrable variety) 7. Northern Irish, which I daresay may also come in many varieties but I don't know anything about them. There are also minor accents which don't really fit into any of those categories, notably Brummie, Geordie, Scouse and Multicultural London English, as Ahmadi mentions.