Thanks Catlady and Linda257 for your replies!
I had a search on some birdseed websites. Couldn't find anything on linseed and rape seed.
Reallywildbirdfood.co.uk says hemp is eaten by greenfinches, woodpeckers, nuthatches, buntings and bullfinches.
Vinehousefarm.co.uk says canary seed is eaten by house sparrows, siskins, many species of finch and buntings.
Peckamix.co.uk says safflower is eaten by bullfinches, chaffinches, collared doves, goldfinches, greenfinches, house sparrows and nuthatches.
The issue is I've never heard from anyone's personal experience having success feeding any of these seeds. And lots of birds just seem to go for sunflower hearts over any other options :/
Maybe I should buy the smallest possible bag and a small birdfeeder just to test one of these out...