Most of Europe used to be Celtic, maybe we should hand it all back to Ireland?
The situation is a bit different when you have a genuine terra nullius being settled, the point of contention being the lack of continuous inhabitance (~reversion to terra nullius) by the Spanio-Argic-Platinates vs the current British inhabitance. Plus, most ethnic english people are still far more genetically celt than latin/ north german/ scando/ scando-norman.
But yeah, the claim is silly, flawed, has been fought over already, and is largely posturing for domestic benefit.
Query: If the claim is for the Spanish empire, doesn't that mean we should hand them to Spain? I know they apparently inherited them. But, you know.
Spain's a particularly interesting case, as they complain about Gibraltar while keeping an enclave opposite in North Africa (Ceuta) which really ought to be Moroccan by their own logic.