There is a collection of 16th century shorthand contained in the Carlton Collection, at the London University Library, and I wonder if anyone has had occasion to view this.
At this point it's fairly important to discover whether or not these symbols were used in any other shorthand samples outside of England as well. If not, English authorship is a very good possibility.
My guess is that the symbols for the script were selected as a cover, so prying eyes would pass over it as a shorthand document on herbs.