Oof, yeah 2011 feels old as hell considering everything that has transpired. Even though they "update it every year based on surveys", the errors are bound to accumulate. But the NIMS data sounds even more fishy as it counts both emigrees and double counts people with multiple GPs.

Here is an interesting graph illustrating your point - the ONS and NIMS data are so out of whack that you get diametrically opposed results using one or the other.

IDK, maybe it would be prudent to look at some better organized countries data to get a clearer picture? :)