Buzz, some hints for finding British stuff.
I had parts books for bikes I worked on a lot. A lot of the better sources reference their stuff by part number. Example, a BSA A-65 clutch basket is 15568-3230. I would always include this number when searching. A picture of the worn part helps, too.
Owner's clubs have spares schemes and sources for hard to get items. It pays to join these. Also, there are often model specific web forums, too.
Pay Pal makes overseas buying much more secure and easier. A currency conversion is easily done when you order. Do not be surprised if the seller charges you a few dollars to use Pay Pal. Usually I pay it and consider it part of doing business.
I hope this helps.