Real BrE pronunciation

You will probably discover that there is actually no such thing as a single pronunciation of either BritE or AmE, in the strictest sense; both have significant variation in the sounds involved within their category. English-speakers, by convention, expect some consistency, but a certain amount of variation is perfectly acceptable in most contexts.

To illustrate how this affects the answer to your question, you doubtless know that anyone who was born within hearing distance of Bow's Bells, the bells of the Church of St. Mary-le-Bow, in London is considered a Cockney, and that Cockneys have a distinct and different accent (as well as their own idiom, Cockney Rhyming Slang). This accent is, to some degree, shared by near neighbors who are not strictly Cockney by that definition. In at least one stratum of society, the accent continues but diminishes with increasing distance from London for at least a dozen miles.

This is important because your question asks about the accents of London residents (presumably), as well as those living near London, and the problem is that they vary a great deal even within that range. The Cockneys are only a minority in London; other residents (and commuters) have a variety of different accents, depending on their background.

Spagirl's comment suggesting reference to YouTube is a helpful idea, but will not give you any indication which accent should be considered "standard".

Why this should not concern the average person: At one point (you may know this, of course, but for completeness I'll include it), the BBC had an approved accent which they expected their announcers to use. Each announcer, therefore, sounded as if they came from the same geographical area, class, education, etc. The BBC's objective was for their radio broadcasts to be as comprehensible to as many different listeners as possible, since for many of them, particularly their overseas listeners, English was not their first language.

The good news is that this became a politically incorrect policy, since it appeared to suggest that one accent (and by extension, the related geographical area, class, education, etc.) was superior or more acceptable than others. As a result of public pressure, the BBC changed their policy regarding the accents of their announcers to allow more regional variation, and now has announcers with a variety of accents, including accents not originally not from the British Islands, such as those of some African, Asian and Caribbean nations.

If you are interested in recreating a particular scene, for documentary or dramatic purposes, it may still be important to perfect the accents. For this purpose, I recommend finding a relevant broadcast from British broadcasting.

For example, the world's longest running television drama, the British soap opera Coronation Street features a variety of accents that are genuine and unfiltered. I regret I can't offer you guidance on a streaming source in Japan.

Another more famous example is the series Masterpiece, produced by the BBC. This series encompasses multiple dramas set in a variety of regions.

The Endeavour television series may be particularly useful, because it pairs a Londoner, Detective Inspector Fred Thursday, with a white, male, upper-middle-class, well-educated Englishman, Constable Endeavour Morse. The contrast in their accents may be informative.

The series is a sequel to an earlier series, Morse, which depicts Constable Morse in his later years as a Detective Chief Inspector. In that series, his assistant is Sergeant Lewis, who is a Geordie* from the north of England, with an accent to match.

Finally, after the actor who played Inspector Morse retired, Lewis became the protagonist in a spin-off series, and this time it's his assistant, Detective Sergeant James Hathaway, who is the white, middle-class Englishman with an Oxford education (and accent to match).

These 3 television series, therefore, offer a surprisingly convenient juxtaposition of London and Geordie accents versus an upper-middle-class/well-educated English accent.

*A Geordie is a person from the Tyneside region in the north of England, i.e. the opposite banks of the River Tyne, comprised of the northern part of County Durham, and the southern part of Northumberland.

As a note on American pronunciation, the letter t between vowels is pronounced as a "t" sound in some cases, and as a "d" in others. For example, butter is pronounced "budder", not "butter" in AmE. This holds mostly for short initial vowels and vocalized following vowels, so "fladder" for flatter, "bidder" for bitter, etc, but "mute" for mute, and "late" for _late. Later, however, is pronounced "lader" in AmE, since the e is now vocalized.