Do you want to speak English with British accent?!
It is very simple!
You should find a British teacher for your self!
But if you can not find a British teacher, Don't Worry!
Here I will introduce you a very good website for learning English with British accent!
It is https://www.britishcouncil.org/ which helps everyone to learn English.
In this website you can find very useful podcasts in different levels.
Click and visit here: https://learnenglish.britishcouncil.org/en/elementary-podcasts
Then start listening to the podcasts regularly.
The Most important thing about these podcasts is that all of them are included with transcripts!
So you can download podcasts into your mobile phone and read their transcript as well.
Cheers!