several ways
hit google earth and see if there are any parks with ponds or lakes nearby
can you afford to get a cheap second car and run it as a wam car for driving to and from muddy places
i have a car thats cheap to insure as a second car, took all the carpets out drilled holes in the floor pan in a few places to let water out, i can drive that completely covered in mud or soaking wet
or loose the housemate!!
btw, where in england are you
(edit, seen other posts, you still in norfolk? my nearest river mud is the thames, its really nice, driving covered head to toe in mud is quite sensual