In reply to Mike Peacock & others:
Interesting suggestions. I hadn't thought about Anglesey.
The Severn estuary I'm quite familiar with and yes it's fairly untrodden for good reason!
My own benchmarks for wild coast are in Pembrokeshire, from Cemaes Head to Fishguard, and North Cornwall from Widemouth Bay to Tintagel.
But really there's remarkably little coast in England and Wales without farmland, villages, caravan parks or industrial sites right up to the edge.
I might try Exmoor.