The Severn Bore
During our last trip we managed to see the Severn Bore once in the dark and once during daylight. Even though the one I filmed was a smaller one (3* out of 5*) it was still really impressive, considering that it was near Newnham, where it starts picking up. The Severn Bore is one of the biggest in the world. You can read more info here.
Here is a 5* Bore I found on YouTube: