On WikiTree I use the citation that FamilySearch creates, such as:
England and Wales, Birth Registration Index, 1837-2008," index, FamilySearch (https://familysearch.org/pal:/MM9.1.1/2X3W-R9J : accessed 22 February 2015), Joseph Dobson Welch Ackroyd, 1865; from "England & Wales Births, 1837-2006," index, findmypast (http://www.findmypast.com : 2012); citing Birth Registration, Bradford, Yorkshire, England, citing General Register Office, Southport, England.
which is ok as you can follow the link. In my offline tree I use:
E&W Birth BMD Q4 1938, Vol 7b Page 678, Repton District, Jane DOE mother nee SMITH
which is enough if you then want to order the full certificate later.
Where I have the certificate (original one or a recent GRO ordered copy), on WikiTree I use: Death Certificate QBDAA 446496 (William Ernest Cook) (and if i remember, the date of the certifiacte, not the event).
Offline, I use:
E&W General Register Office, Birth Certificate BXCH123456 (date of certificate), Jane DOE. I never share an image of ther certificate, but will transcribe the whole thing.
Hope that helps,