Ignore them.
You don't need a "back up". Your "back up" will be easier universities to get into, not different courses. Apply to study law at all your choices if that is the subject you are set on, but if you are not confident of getting into all your choices (if they are really competitive) then add one or two lower universities which you know you definitely would get into.
It makes no sense to have a backup course because if that course is where you end up and it is not the course you want to do, do you really want to spend 4 years (assuming you want to go to a Scottish uni) studying it? The only subjects having a backup course makes sense for are medicine, dentistry and vet med.
I also don't understand how you would be able to do this with only 1 personal statement on UCAS. You can't really be talking about law and then something like lit or history.