Here's why.
The name 'British' can mean anyone from the Top of Scotland, across to Northern Ireland, down into England and over to Wales. I am sure you would agree if you have heard a Scots, Welsh, Northern Irish or English person talk, that their accents vary by a huge degree!
Whenever I hear someone on TV (Usually from the USA or Canada) make reference to a British accent, I am 99% certain that they are thinking of the 'English' accent. I don't think I have ever heard someone call a Scottish accent 'British' or an Irish accent 'British'.
Furthermore, I am well aware that there are also regional differences in accents all across the USA and Canada, however (and feel free to correct me) I would say that it is quite valid to describe an accent from Texas, or from California as an 'American' accent. Or when listening to a Nova Scotian, or British Colombian talk, I think it is fair to say that they have 'Canadian' accents. In fact both the US and Canada would probably qualify for 'North American' accents given their similarity.
In the UK however, because each region is far older in comparison to North America, these areas have developed far more distinctive and separate accents which you simply cannot put in a bag and call it 'British'.
Maybe if you mixed an English accent with Scottish, add a dash of Welsh and a pinch of Irish, maybe then, we could describe that, as a British accent, but I have never heard one. What about you?
