In the UK, it isn't necessarily accent per-say.

In North England and Scotland, we pronounce words using short vowel sounds (bath, castle) whereas in the South, they use long vowel sounds (b-are-th, c-are-stle).

Not being able to easily pick out the vowel sounds makes it much harder for a non-native speaker to understand.

The so called "BBC English" that you probably learnt is based on a Southern England 'home counties' pronunciation.