I noticed that the diphthong /aʊ/ is pronounced by most British English speakers as [ɑʊ] (I may be inaccurate here).
You can see the vowel /a/ on the vowel diagram below:
I understand this diagram quite well and the vowel /a/ is located at left bottom, so when I pronounce the diphthong /aʊ/ with the vowel /a/ (as shown in the diagram), it doesn't sound like the one pronounced by native British English speakers at all. But when I pronounce the diphthong /aʊ/ with the BATH vowel (Southern British English BATH: /ɑː/), it sounds quite normal.
Also listen to the following YouTube videos you will observe:
(copied with exact time)
Is the diphthong /aʊ/ generally realized as [ɑʊ] in Southern British English (also called "Modern RP")? (I would also be interested to know how American speakers pronounce it generally.)