is that the LCM as in London College of Music (board)? i study at the London College of Music, and i know they have their own boards for grades and diplomas. if it is this, and you do live in UK, i would suggest going for ABRSM diploma. They just seem better to me as a qualification.
on the discussion of it as being something worth doing, yes i would recommend doing it. go to
www.abrsm.org find out some more about the diplomas. if they send booklets from online, ask for one. if not phone them up, or visit the place on
24 Portland Street (cant remember post code) but its in central london near Great Portland Street and near Marylebone. the booklet contains most info you need.
hope this helps.