This is a common confusion and is not true. What is illegal is to dig them up in order to sell them. To dig up a reasonable quantity for personal use, provided they are not a specifically protected species, or belong to someone else, is perfectly legal. The woman from the New Forest who got prosecuted for picking wild mushrooms last year was done because she was selling them, but even then she managed to find a loophole that allowed her to continue. Of course you may not agree that this is how it should be, but that is something else.