Not quite B&Bs, but "The Good Hotel Guide 2008 Great Britain & Ireland" has independent reviews of small hotels and guesthouses - many of which are barely larger than a B&B. The Guide does not accept advertising.

Additionally, some of the listed properties have agreed to accept a voucher cut out of the guide for 25% off one night's stay. I took advantage of this on a trip to Dublin last month and saved 34 Euros on the first night at one of the more up-scale listings (Aberdeen Lodge). The Guide is somewhat sparse in that it only lists properties for which it has received enough recent reviews from its correspondents for its editors to feel confident that the information is both accurate and current - some areas have no properties listed.

I've no connection with the Guide except as a satisfied customer.

http://www.goodhotelguide.com/