Not sure about the law but certainly a good idea, one thing that is also very sensible which a lot of people don't do is fasten the rear seat belts when not in use. the seat belts are much stringer than the seat hinges and heavy thing in the boot hitting the seat backs at force can cause folding seats to fold, fastening the seat belts ensures that the seats can't fold and forces any loose items to the base of the seat.
I would imagine its ok not to have it fixed, certainly I had a car once that just had it sat in a well and wasn't fixed, but don't quote me on legality. as far as I am aware the only physical things that police can check internal of the car are that the battery is secured and that the washers are full, not seen anything about secure spare wheel.
If you're going to drive about with a loose spare wheel then I'd certainly fasten the rear seat belts (the outer ones) so that each seat back is secured in its upright position.