Do You Understand British English w/o Subtitles?

I’m just curious, but do American people or people in other English speaking countries (except for the UK) understand what the characters are saying in movies or series where they speak British English?

I’m a native Japanese who was born and raised in Japan, yet I prefer to watch anime with Japanese subtitles. I recently watched Harry Potter movies and I’m wondering if people can really understand English with a different accent than their own.

I’ve been learning rather American English in school, so I hardly understand when people speak English with a quite strong British accent... (as well as other strong accents. I can understand English with a strong samurai accent tho XD)

If you’re an English speaker and not from the UK, can you understand everything what the characters say in Harry Potter movies without subtitles?