The National Biodiversity Network allows you to view distribution maps and download UK wildlife data by using a variety of interactive tools. One can search for wild food plants and, best of all, it is possible to browse for species (plants, animals and insects) of conservation concern.
Through foraging, the last thing I would want to encourage is for anybody to deplete our natural environment. One thing to bear in mind is that although a wild food source may seem freely and abundantly available it is possible that some of the insect species it supports are themselves threatened or in decline.