Masashi, it's not possible to lock yourself inside a room with some study materials and then come out months later to say "Listen! I have a British accent!!"

We get our accents from the people we speak with. So, if you want any particular kind of British accent, you need to communicate with people who have that accent (and those same people have to tell you when you don't sound natural).

Something else to consider... I'm sure you're quite aware of the accents and dialects in your first language, and you can also identify what a "standard accent" is. It's the same with any native English speaker: we certainly know what a standard accent sounds like, and most of us can use it quite well, even if our daily accent isn't exactly standard.  So please keep that in mind if you want to focus on strong regional accents.

I think RP is your best bet. It's very commonly heard and features in many films, television series, and news broadcasts. There are lots of materials available that you can try to imitate.

The problem with learning an accent like Cockney is that it has a very distinct corresponding dialect. A foreign learner using more standard English grammar and vocabulary with a Cockney accent is unlikely to be taken very seriously. Understand that the Londoners who speak Cockney are a minority and make up only a very small percentage of Britain. Estuary English is a less extreme case, but still features a similar story..

There are videos online on YouTube that can help you e.g. https://www.youtube.com/watch?v=xuB1IdOqOM4

You could consider spending a long period of time in England and you will naturally adopt the accent. Or get an accent coach. Keep in mind that even Americans whom English is their native language struggle to master a convincing British accent. But good luck.