This ghost sign was most certainly painted during the inter-war years, as indicated by the presence of the dash on the 'o' of Hovis. Hovis slogans with a particular emphasis on and featuring the word "health" were also characteristic of that period.
Since bread had had a bad reputation throughout most of the 19th century, bakeries that introduced modern technologies were usually keen on advertising it. For such a ghost sign, see the post about the Hygienic Bakeries in Richmond.
*: the first electric baking machines used in British bakeries seem to have been introduced towards the end of the first decade of the 20th century, once the production and distribution of electricity had become more reliable, but it was only after the First World War that their numbers really increased. Click here to see a picture of the Electric Machine Bakery of A. W. Davis in Herne Hill taken in 1910.
Location: Islington High Street / Picture taken in August 2008