//And given that the latest official figures suggest the UK currently has negative net migration - more people leaving than arriving - it is unlikely to be about numbers,...//
If it was true that more people are leaving than arriving (and I'm not entirely sure that it is but don't have the figures to hand) it may not be about numbers, but there is another consideration. That is "population exchange". What has been happening in the UK for a coupe of decades or so is that people who are leaving are predominantly either members of the professions or skilled workers with a lot to offer their new home nation or they are retired and self-supporting (because many nations will not allow you to settle if you are not). In exchange we see the arrival of largely unskilled or unemployable people who, even if they do work, are unlikely to make a huge contribution to the UK's coffers. The notion that those arriving in rubber boats are architects, lawyers and surgeons is fanciful. If they were, they would apply to settle in the UK via the proper channels and arrive by conventional means.
So, if you're happy to see the UK's population of skilled construction workers, doctors and nurses, together with wealthy pensioners exchanged for fruit pickers, car washers and McDonalds moped delivery drivers, then the arrival of tens of thousand on the beaches in Kent is a welcome sight.