I've checked with dad who used to have a company selling dried edible seaweed. If it's cast, then it's usually the local authority (or whoever owns the land at the top of the beach). If it's growing, apparently you have to talk to the coastguard but they'd probably rather not know!
He made the comment that if you're not sure to ask, go collect some & the person you need permission from is the person who tells you to stop!
One proviso - if there's any kind of conservation work, or it's a nature reserve, it would be irresponsible to collect it as it's part of a valuable ecosystem. I prefer a beach with seaweed on it personally. Much better for wildlife. It is very good garden food, but also a great way to get more stones on your veggie patch!