So, I'm trying to find a source of Blueprints for two British engines; Stroudley's terriers and the SECR P-Class. A traditional google search hasn't been entirely helpful, so I would hope that someone here would be able to point me in the right direction.

Have you tried the National Railway Museum in York, U.K.? It is probably one of the best and largest railway museums in the world, and they have an extensive archive, most of which is searchable online .... http://www.nrm.org.uk/ResearchAndArchive.aspx

Do you need general diagrams, or enough detail to repair one, or enough detail to build one from scratch?

The first can be had cheap--somewhere. The latter are going to be a pain.

Actually I'm looking for full-blown diagrams. NRM might have something in the archives... I'll have to start digging there.
*A general arrangement might be a start...

I would guess what you are really looking for is drawings, as the manor by which they were copied is irrelevant. What you will probably receive is a PDF scan if your search is successful. If you want more that General Arrangement drawings, you probably should be first looking for the Bill of Materials or Drawing List so you know what you are looking for. Once you get drawing numbers, the search should be fairly quick.