lou,
A police caution is not a conviction and therefore you do not have a criminal record. You do however have a police record and an enhanced CRB check will show all convictions and cautions. Generally it will depend what the caution was for theft, assault etc will obviously raise issues with an employer who is responsible for maintaining the welfare of vunrable people. If you were under 18 when you were warned ( cautioned ) this will not show on a CRB check.
You say that you will be doing a nursing course, I don't see why a CRB check would be done by the uni. ?
Maybe one of the other ABers can answer.
Hope this helps.
LMan