Sometimes I watch films or videos in British accent and I don't understand anything. They talk too fast and link words and don't pronounce a lot of letters. I watched some YouTube videos to learn the British accent such as "master the British accent" series and some BBC British accent videos and a lot more and I still don't understand anything. So any tip to help me understand more. Could you suggest any YouTube videos that would help me. Thanks for answering.

Time and practice. I've not had issues with speakers from England, but my neighbors who come from Scotland are especially difficult for me to understand. However I find with time and concentration I get better at it. Same with speakers from India - many have an accent that can be quite difficult, and in addition they use phrases that can seem bit odd to American listeners, so it takes some effort to clearly understand.

Is English your first language? If English is your second language, then part of the issue may also be trying to translate the words as well as understand the way they are spoken and used. This is common and, as ebaines said, takes time and practice.