Without being negative, one problem with edibles is that they appear at different times north to south, and to a lesser extent east to west, and that's without the seasonal variations from year to year, so you may just have to go with a general period expressed in months. This does someowheat reduce the value of the guide, but only slightly. So, let me provide one:
St George's mushroom. A good edible, difficult to confuse wth other species. Traditionally available on St George's day 23rd April, but more realistically available in May.
You might also note that some species, such as nettles, are available most of the year, but have periods when they at their best (springtime for nettles).