Examples of unjust laws in the UKWatch
Woman are not rapists, they can't legally rape someone because of the wording.
Fault divorce - As it stands, to get a divorce you need to evident one of the five facts; adultery, behaviour, desertion, two years' separation (if the other spouse consents to the divorce), or five years' separation (if the other spouse disagrees).
i'd say choose a law you wish to change in that specific area, as choosing any law whatsoever might read a bit weird if it's not relevant to you or your application. being in mind also that your argument for why it is unjust is everything. interested to hear what your application is for.