Most Linux machines are configured so that the kernel occupies the upper 1G (0xc0000000 - 0xffffffff) of the 4G address space and processes use the lower 3G (0x00000000 - 0xbfffffff). However, some machine are configured with a 2G/2G split, with the kernel occupying the upper 2G (0x80000000 - 0xffffffff) and processes using the lower 2G (0x00000000 - 0x7fffffff).
The prebuilt UML binaries on this site will not run on 2G/2G hosts because UML occupies the upper .5G of the 3G process address space (0xa0000000 - 0xbfffffff). Obviously, on 2G/2G hosts, this is right in the middle of the kernel address space, so UML won't even load - it will immediately segfault.
The fix for this is to rebuild UML from source after enabling CONFIG_HOST_2G_2G (under 'General Setup'). This will cause UML to load itself in the top .5G of that smaller process address space, where it will run fine. See Compiling the kernel and modules if you need help building UML from source.