does anyone know of a UK charity who can use floppy discs?

Answer 1

Agree with Steve but don't forget to delete any content by formatting the floppy first.
Go to My Computer , put in the disc, highlight floppy drive then Right Click and then Format - wait until its finished and then its clean.

Answer 2

You want to donate a few million blank floppies ?

Take them to any of the major high street Charity shops = all the big ones 'network' the goods that come in around their own shops across UK ...