UK Cover Letter
A British covering letter is an American query letter.
Jeff
From what I've encountered on British literary agency websites, the cover letter is something more basic than the query. They don't want the equivalent of a back page blurb, but only the essentials.
One of the best UK agencies, Conville & Walsh, has a dedicated reader for debut novelists, David Lewellyn. From a link on their website he says the following about the cover letter:
"The covering letter should be no more than a courtesy letter, containing a very short, two-line summary of the manuscript, reference to any other published or unpublished work and relevant personal details. ‘Relevant personal details’ is not meant to include the following: information on the author's writing history from childhood onwards; lists of favourite books and authors; what the neighbours, friends, children and extended family, think of their work; or any obscure information which will hinder the Reader from starting the assessment of the manuscript in hand. Be concise and be professional. The author may complain that this does not allow for an individual approach and does not allow him/her sufficient opportunity for self expression. The manuscript in hand will do that, so let the Reader get to it, as soon as possible. Allow time for the manuscript to talk for you."
http://www.convilleandwalsh.com/index.php/literary/submissions/
So it's a different sensibility than the American approach, emphasizing the Manuscript's writing rather than a pitch. Hope that helps.