Again - am not saying this is good - although as PP mentioned up thread - in part this system was allowed precisely because NHS werent sure of the numbers they could get in the first place. Plus seeing as plenty of British doctors leave the NHS - doctors leaving the profession isnt exclusively an issue with IMGs.
I have also lived in Switzerland which basically imported lots of trained medics from Germany because it's cheaper, Germany gets them from elsewhere in Europe e.g. Romania where they especially train German speaking medics to work in Germany. In Switzerland - locals dont see medicine as such a great career to get into and dont. In fact, lots of countries choose to import rather than train medics - because it's cheaper than training them and you can create a much more flex job market in a profession that isnt all that flexible in terms of producing a brand new professional tomorrow.