Gaziano & Girling. The ribbons can be inserted before an event and removed afterwards, lending a new look to an existing shoe.
Shoemakers buy these in by the box, but I don’t know of anywhere in London that sells them retail. (I acquired my pair from Tony and Dean at G&G.) If anyone does and can recommend a supplier, please let us know.
UPDATE: Cleverley normally sells them, priced at £10, but they are out of stock at the moment. Thanks for your comments.