RP is a standard, neutral pronunciation. It's the British equivalent of the 'accentless' General American' pronunciation which is favoured by newsreaders, for example. English speakers in the British Isles have dozens of different accents, some of which are very different from standard English and very difficult for outsiders to understand. As a foreign learner, you could meet someone from Glasgow, Newcastle, Liverpool or Belfast and possibly not follow a single word that that these people are saying. So it would be a very strange thing for you, as a Japanese person, to learn to speak with one of these regional accents. RP, on the other hand, is neutral. Whilst it's true that RP is closest to the accents of southern, especially south-east England, it does not have any particular regional association. It is 100% clear to anyone, no matter what their origins are. A train or airport announcement, a recording in lift, or a radio advertisement is likely to be in RP anywhere in the UK. This is not the case with any other accent. Not everybody in the UK speaks with an RP accent, but a large proportion of educated and articulate people in the UK speak with an an accent which is a version of RP, even if it's not exactly the same as that of the lady on the Cambridge Dictionary website. An educated Scottish person will have slightly different vowels, and will pronounce 'r' sounds at the ends of syllables, for example, while an educated Yorkshireman might use 90% of the same pronunciation as RP, but will rhyme 'pass' with 'mass'. But the point is that RP in some form is familiar to everybody in the UK, and completely understandable to everybody. That's the reason that you should use RP as your model.

Su.Ki. said almost everything about RP. Well, all I can add is that Received Pronunciation is also referred to as BBC English or Cockney. Cockney is the name of Londoners' accent.