Hello All,
  I suspect the UK modellers know this site but the rest of us have to do some searching. The City Archivist in Glasgow pointed me to this link and it is very good and useful for plans searching. This is the search point for the National Records Archives
http://www.nationalarchives.gov.uk/nra/default.asp
It is the registry of archives (e.g. holdings) and searching by corporate name ( i.e. Vickers) brings up links which you can click on to get the various sources of shipbuilders plans. Happily, NMM doesn't have everything and this may help some who need plans  from elsewhere. There are email links and regular links to various repositories. Tyne and Wear seems to hold a lot of material, for example.
good luck
dave