Hi there..sorry for the delay...but we just now received the below information from Black & White's parent company, Diageo:
From looking at the photo of your Black & White bar mirror, the archive department dates it to c1975. The text font and the style of the Royal crest, as well as the way the two dogs are positioned is in keeping with the way the bottle itself was labelled specifically between 1975 and 1979.
We do not provide valuations, however if you were looking for an expert opinion, it would be worth email an image and a short description to BBR Auctions http://www.onlinebbr.com/contact/
This is a British auction house that specialises in all kinds of beverage memorabilia. Similarly, if you can find any dealers in your local area who specialise in pub and drinks industry items, they would be best placed to give you an accurate valuation.