Most formal Police cautions normally expire after 5 years, however, not all Police Forces operate the same way.
You should make contact with the Force who cautioned you and find out if your caution has been deleted. This is normally done by application via a special form they should have for you to fill out.
However, if your private hire cab application requires you to disclose certain details, i.e. regarding any convictions / cautions, it is incumbent on you to answer honestly.
A Police caution is NOT a criminal conviction, and details are only held on the likes of the Police National Computer.
Hope this helps.