If you don't mind getting south of the Humber you might consider North Coates - thread on it in beaches(UK) or, farther south Theddlethorpe has been mentioned to me, north of Mablethorpe but I hope local users of these beaches on this site and in Yorkshire can advise you.

Like all the beaches on the east coast in this area Theddlethorpe is a very long (almost never ending) and wide beach. I've not been to the actual unofficial nudist area of the beach, looking at the write ups it can be difficult to find and when I google'd it again I found a lot of guy meet chatter in the dunes (which put me off), but as I say its a very long beach and some days hardly anyone there - so I just found a secluded area in the dunes where I could easily see people coming and stripped off; only problem is its a long way to the sea.

Yes, Theddlethorpe is a very nice clean beach that stretches for miles and miles. If you check out Google Earth you will find a place called Churchill Lane which is where you need to head for. Park in the car park there and walk along the cutting for about three hundred yards. When you arrive at the beach, turn left and walk along the edge of the dunes for about another three hundred yards. Anywhere along here is safe for nude sunbathing but as it is not official, you need to show some respect to dog walkers and the like. It is just so quiet and peaceful, I love the place.