‎‎You can master British accent with the apps based on RP:

Elocution Lessons, Get Rid of your Accent UK1, ‎Business English Speech and ‎Fluent English Speech, and Power, Pitch, Pace, Pause.
There are 2 accompanying video courses: Get Rid of your Accent Part 1 and Get rid of your Accent Part 2.

The same apps are also available on Google Play. The functionality is ideal: read, listen, record and compare your speech with the model. The apps have:
1. Spelling variations for all English sounds
2. Mouth diagrams to help you create the correct speech organ positions in words, phrases and poetry
3. The list of difficult sounds typical for your nationality
When starting your practice with these apps, go to “How to use this app” and click on: the difficult sounds typical for your nationality, then click on your nationality to work effectively with a more focused approach.