Mass immigration ain't a problem: the Aristocracy and the Ministry of Offence
own tons (about half the country) of land which could easily be
used to grow food sustainably for incomers and locals.

The issue, therefore, is land and the unwillingness of those
who own that land to allow others to use it.

Current housing stock occupies a measly 9% of all land in the UK.
In England it is probably 11 or 12%, but the point of this is that there
is more room generally in Scotland! Also, thousands of houses are
unoccupied as the market is unwilling to buy or rent them, despite
plenty of demand from English residents.

The issue with immigration is about incorporating new people
into the capitalist market, which capital benefits from in the short
term by reducing wages but loses out in the long term when capital
raised by migrants is invested in their native economies.

Swamps of migrants would not affect the market, except for the
educated minority which can and is seen as a capital inflow into
the global north, as literally millions of people coming at once
could not possibly be housed in major urban areas or be provided
with regular jobs.

I try not to think of things as absolute, and I certainly don't want the
countryside paved over, but was making a point that a good two
million or so could "live off the land" as it were.

The only threat mass immigration poses is a threat to private property.