What is this strange sea creature we found on the beach?
You have a Dosima: Also known as a Buoy Barnacle. A gallery of observations of these can be found here:
https://inaturalist.ca/taxa/462188-Dosima/browse_photos
They are found in the coastal UK and Spain area, New Zealand, and east and west N.America:

(source: iNaturalist observations)
These barnacles attach themselves to things that float, and if they don't have access to one, they will grow one themselves. Your image looks like five individuals attached to a central float that one of them (or a dead relative) constructed.