One of the drivers of the influx of workers from the EU, particularly the Eastern EU, has in the past been the high value of sterling in relation to the euro. In the eurozone a euro buys more or less what a pound buys in the UK. But if you are from the euro area and you work here and send money home, there has been a handy bonus because the converted pounds produce more euros. With the decline of sterling since the Brexit vote and the increased strength of the euro economy, which is now growing faster than a UK mired in uncertainty about the Brexit deal, the bonus has dwindled to next to nothing. Many analysts expect parity when we reach the actual Brexit moment.
So the fact that the net migration numbers are falling has probably as much to do with the currency as it has to do with politics.