Mark, Mr G, your welcome
All of the collections are on the NRM web site to browse through, not all are on fiche though.
Drawings and papers from railway works and companies - Research & archive - National Railway Museum
For BR stuff scroll down to the Companies section and British Rail, download the pdf files and dredge through them, that'll give you a list of what you think you need, then contact NRM and set up a date to visit and they will get the files from the archives for you if they are paper ones, if they are on fiche they tend to do it on the spot if you don't have too many to browse through. Both the GA for the Britannia and Standard 5 were on fiche as was the Battle of Britain frame, stays and nameplate drawings and some of the A3, A4, V2, A1, A2 and A1/3 I've collected over the years, but an awful lot are still on paper.
For other works just scroll further down and download the pdf archives or click the
Selected Drawings link in each works section for a sample of drawings from those collections.
They seem quite relaxed with regards you photographing paper drawings, I've not tried it yet but others seem to have had no problems, I usually go for CD digital copies but might go for paper ones on the next project, certainly for the GA and frame drawings, the stays and detail ones I might just try and photograph or get on CD.
All the best
Mick D