Mutualistic Symbiotic Parasite is a more accurate description. Both entities closely connected, producing a mutually beneficial relationship for both parties.
'Tapeworms cause health problems around the world and can even kill since they rob us of nutrients, block our intestines, and take up space in organs that stop them from functioning normally.'
A tapeworm isn't a mutualistic symbiotic parasite though. I dispute your analogy as the UK has received huge benefits from it's membership in the EU. IMO, the parasitic relationship is not one of commensalism but of mutualism .