Just as you can take into account the number of runways vs the number of movements, you can also take into account the number of movements per ATCO, the number of testicles per ATCO and the number of moron pilots per 100 movements. These things also have an effect on how busy someone will be and how "good" a controller would be in each different environment.
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battle hardened original residents of these Islands
being the only ones able to do the job is laughable when some of these battle hardened residents have been found lacking at other locations around the world. Given time to learn local nuances, I think the majority of ATCOs from reasonably busy airspaces would make the grade in the UK, if only given the opportunity.