With high salaries, it sort of cancels out this problem.
(a)
This would be hard to enforce in a democracy? Why only dark-skinned people affected?
Looking at this, the UK population density is not particularly high
http://m.statisticstimes.com/demographics/countries-by-population-density.php
So I doubt the citizens in this Empire would see it like this and unlike many countries most of the UK is inhabitable.
Some parts of the empire with large population densities and populations today.
Bangladesh 1,252 people per square kilometre
Hong Kong 6,690 persons per square kilometre
This will be tried. The locals there will be trying to stop it too which might help make a principal of restriction of movement.
Large-scale migration to the UK from SE Asia started in 1947, the cost of movement did not stop it but government action. With today's cheap airfares and transport, cost would not be much of a problem. When I did a search on google, I found aeroplanes price from New Delhi to London for about $1,500. Many Indians and Black Africans can afford this one off fee.
It would work both ways, people living in England might go to these places soon.
Also, it conflicts with what you said at (a) above.