Hi Islander,
I suggest you contact your GP surgery again and ask them to spell out in detail the exact the name of this supposed "Home Office Document" they say they require, cos it seems to me that almost all of members of the forum that have already registered with GP's didn't need anything other than passport and maybe marriage certificate. Then if there is such a document that no one on here seems to know about, you could try emailing the Home Office at ukbapublicenquiries@ind.homeoffice.gsi.gov.uk to see why you haven't been issued with one.
Iain.