The Thames, the Severn... and then there's more competition for rounding out the set. Avon? Trent? Great Ouse? Tyne? Ouse? Wye?
This was apropos of a very interesting subject: a third-hand critique of Neil Gaiman's Englishness, and how he includes no river gods in American Gods, and the role which rivers play in Americans' conception of their country. (It's true. I picture the Midwest as a watershed, my home state bounded by the Missouri and the Mississippi.) The discussants then mildly contested this claim, as England is a country chockful of rivers, as is shown by how easily most of the country would be submerged if the waterlevels rose. But that's just the point. American's rivers are on a scale so much bigger than those of England. They carve the landscape on a much more momumental scale. England's rivers have their moments of vastness, but as they mingle with the sea and become part of the coast.
Let me illustrate. Here's a photo of the Arkansas River, a tributary of the Mississippi, and the sixth-longest river in the US. It's not a river I think about much, I have to say. This is a photo of it a good two hundred miles from the Mississippi. (Photo taken from Petit Jean State Park.)
Obviously size and influence don't necessarily correlate in terms of rivers' influence on their countries, and England has been very much formed by its rivers, especially before the railroad, when cargo transportation was still easiest on boats and ships.