You've got a great area there - Mrs 3BM grew up in the Beacons, when we got married there and have had family events that way booking B&Bs can be tricky sometimes due to lack of rooms.
Both my parents, long since gone their separate ways, have ended up running B&Bs (strangely enough) and although hard work they enjoy it and have met really interesting people through it.
But, as has been said before, quality is the key to success and repeat business.
If you do it - Good luck!!
Edit: If you want LD I could PM you their websites (don't want to use Admin's site as free advertising!) if you want to see how they do it. Pop's place is in Normandy and Ma's is in the Peaks (she's got a 5* rating, so it's a nice place)