Sole Inventor

While not your conventional blue plaque, this simple framed sign in Holborn serves its purpose,

and I quite liked this Covent Garden shopfront, with its charming sign (even though it looks like a floating gravestone):

Run by the Collins family (who named every firstborn son "Fred") for seven generations before the most recent Fred Collins retired for health reasons, the shop is now owned by a friend of the Collins family, and the basement still contains the cast iron pots which were used to make the glue advertised in the sign.