So long as the engine number matches the V5 then you should be fine.
If you think about it you could have fitted parts from a hundred different cars when the car was in daily use. It wouldn't have affected the vehicle identity.
If it makes you feel better, get all the parts together, fit them to your donor and then take them off!
As you say individual parts without a serial number are impossible to determine the origin of.
The SVA guys like pictures especially the seat belt mountings (Which are out of sight, of course) but the main reason they ask is to see what your garage is like.
If you're claiming Amatuer built status and you seem to work in an industrial unit with fifteen similar cars in the background, they might be a little dubious.