I suppose the other, relavent, issue is that, dispite the fact you may own the house and garden, quite often you don't own *all* of the garden. (There's normally a section within the front garden which has services underneath it - this'll be the bit you DON'T own)
You're not permitted to dig up what's not yours . . . refer to the house deeds and the outline plan of your property and surroundings.