My research was mostly focused on Ireland, but I also looked at some UK material.
A great place to start is the Wellcome Library and Archives in London, particularly for primary sources. They have collections of nutrition material: https://wellcomelibrary.org/collections/archive-guides/food-and-medicine/
They also tend to have good short essays / blog posts online about various topics, and some of the material is digitised (if you are not able to visit in person).
A few reading recommendations on health promotion from a theoretical perspective: