I don’t know why you say the Iberians weren’t celt.
The Iberians were a non Indo-European group, the last rements of which exist as the Basque. They are sometimes thought to be true decendents of CroMagnon Man in both genetics and language. The Celts are largely considered a cultural group from central Europe and all forms of their language are Indo-European. In Britian there is a disconnect between culture and genetics where some populations are genetically close to Iberians but culturally Celtic.
If your’re interested in learning more, there is a sub field in archaeology called archeogenetics.