Most guides to foraging start with a three page caveat list of legalities, by laws, and death by fungus warnings.
I'll note only this.
If all you're picking it for is to eat right now as trail food, or taking a small bagful of something to make a meal accompaniment out of, or maybe enough fruit to make a jar or two of chutney from, and you're doing it with nothing more than your hands for picking, and a bag to put it in, and you’re stood on a public right of way whilst you’re picking, and it is clearly not a commercial crop...then even the most belligerent claimant on the land will find it difficult to do anything other than ask you to move along if they feel you’ve overstepped the mark in picking what they believe to be theirs. If the land is clearly private, or commercial, and signed as such, then you should respect it as such.
On the subject of identification – the things that are generally worth foraging for, due to their abundance, are leaves, berries, nut, and creatures that are little more than the wild cousins of the farmed varieties of what you can buy in any supermarket or farmers market, and with one of the many handbooks and internet photo blogs to compare it visually to, and the sense to “try a bit” before you make a 3 course meal out of it, you haven’t much to fear from the British countryside.
My favourite identification field guides are Richard Mabey's Food for Free and John Wright's Edible Seashore. If you wish to become an expert on the more peculiar and rare wild edibles, then there are not a shortage of people running courses – my blog has some links to a few other bloggers running them: