While it's correct that myXelement.Attribute("URL")
would require the correct case, there's nothing stopping you doing
var attribute = x.Attributes()
.SingleOrDefault(xa =>
string.Equals(xa.Name.LocalName,
"Url",
StringComparison.InvariantCultureIgnoreCase));
Wrap this up in a suitable extension or static helper method and your consuming code shouldn't be too bad.