Scotland's estimated population as at 2009 is 5.2 million, lower than in 1961. Indeed, the Scottish population was 4.8 million a century ago – in 1911. Over the same period, the population of England has increased by more than 50% – from about 33.7 million to about 51.5 million. In 2003, England had a population density of 383 persons per square kilometre, compared to just 65 persons per square kilometre in Scotland.
The moral I want to draw from this is that although England might be sufficiently crowded, and have sufficient internally-generated population growth to have an interest in restricting immigration, the same does not necessarily apply to all parts of the United Kingdom.
Now, of course, as matters stand, people have freedom of mobility within the UK once they are here, and immigrants tend to congregate in certain parts of England where the jobs are. But would it really be so infeasible, for example, to make it a condition of being granted a residency visa that one were resident in Scotland, say – having a penalty of deportation if one were found to be resident in another part? Why don't we have an immigration policy that directs immigrants to the least densely populated (indeed, in some cases under-populated) parts of the country and parts that have not had population growth?