I acknowledge that, as the GRO site states, "As long as there are no details about living individuals there is no problem about publishing the images of the certificates on your family tree website. Any modern certificates would be subject to the Data Protection Act, and would need the
permission of any living named person prior to publication."
This doesn't apply to images from Ancestry (?) but that's for another day.
As far as GRO Certificates are concerned, any "special" number on them identifies the particular piece of paper, which is a certified copy of the original entry in the books, and the only foolproof identification of the actual entry is the Quarter, District, Volume and Page where it can be found. So I think I am on safe enough ground if I support basic biographical details such as birth/marriage/death with a reference to, for example, "Marriage Certificate UK GRO Marriages Jun 1905 Wandsworth 1d 912". I'm just starting in to adding some 2100 names to my/our/the tree, so I need to keep things as simple as possible to start with (fact + source only, no digressions, no images online). The honour code works both ways and you can trust me to enter only data that can be supported (though I will obviously oblige if anyone needs to see a scan).