Looks like UK presales didn't indicate much. Sold over 30% on launch day vs its predecessors. Truth is Samsung's main turf is Asia, its home continent that makes up 60% of the global population. But the fact the S5 outsold twice as much than the S4 in Europe means Samsung is doing very well in other places and will continue to sell well.
Personally after seeing the S5 again like 10 min ago, I think I have been more impressed with it than the S4 from first impression. I like the dimensions of the S4 better, but Samsung improved on the build quality and everything else. I see zero lag. Maybe it is from KitKat, but it is smooth. Everything from the camera seems improved. Should be the hot-seller smartphone of the year next to the iPhone 6. But it generally has been that way since the Galaxy S introduction back in 2010. Either the latest iPhone or Galaxy S sell the most over any other flagship smartphone product line every year.